Image displaying method and image forming apparatus utilizing a reversible image display medium having a high resolution image display

ABSTRACT

A method and apparatus capable of displaying a high-quality image using a reversible image display medium in which a cell(s) formed between two substrates accommodates at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities. in image display, an electric field strength of, e.g., 0.3 V/μm to 3.0 V/μm is applied to the developer. An oscillating force (e.g. oscillating magnetic filed) is applicable and the oscillating force is substantially stopped during application of the electrostatic field. After completion of the electrostatic field, the surface of the medium on the image observation side may be charged to a potential holding the displayed image. Optionally before image display, an alternating electric field may be applied to stir the developer for initialization of the medium.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent applications No.2000-350197, No. 2000-350225, No. 2000-350231 and No. 2000-350233 filedin Japan on Nov. 16, 2000, respectively, the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image displaying method and an imageforming apparatus, and particularly relates to an image displayingmethod and an image forming apparatus utilizing a reversible imagedisplay medium, in which image displaying and image erasing operationscan be repeated.

2. Description of the Background Art

At present, image display is performed, e.g., in the following manners.A person uses a pencil, a pen, paints or the like, and manually writesor draws characters, pictures or the like on an image display mediumsuch as paper sheet. Also, a computer, a word processor or the like isused to display text, graphics or the like on a display such as a CRTdisplay, or output them on a medium such as a paper sheet via a printerfor display.

A copying machine or the like may be used for producing duplication, ona medium of paper or the like, of the texts, pictures, graphics or thelike, which are produced on the medium of paper or the like by a personor by a printer. A facsimile machine may be used for sending suchcontents (texts, pictures, graphics and others) prepared in the abovemanner for producing duplication on another medium of paper or the like.

The above image display, which is performed to display the texts,pictures or the like on the image display medium of paper or the like bya pencil, pen or the like, or by an image forming apparatus such as aprinter, a copying machine or a facsimile machine operating in aelectrophotographic method, an ink-jet method, a heat transfer method orthe like, can achieve clear image display in a high resolution, and thuscan achieve easy-on-the-eyes display.

However, it is impossible to repeat display and erasure of the images onthe image display medium of paper or the like. In the case where thepaper is used for writing characters or the like by a pencil, thecharacters can be erased by an eraser to a certain extent. However, itis difficult to erase completely the characters or the like written inan ordinary density, although it may be possible when written in a lightdensity. The medium of paper or the like can not be reused except forthe case of using the rear surface of the medium, which is not yet usedfor the image display.

Accordingly, the medium of paper or the like bearing images will beabandoned or burnt when it is not longer required. This results inconsumption of a large mount of resources. The printer, copying machineor the like also consume consumable products or materials such as toneror ink. For obtaining the new display medium of paper or the like aswell as toner, ink or the like, energies and resources are required forproducing them. This is contrary to the current demand for reduction inenvironmental loads.

In contrast to the above, the image display by a display such as a CRTdisplay can repeat the image display and the image erasure. However, theresolution, clarity and precision of images are restricted, as comparedwith the images displayed by the printer or the like on the paper mediumor the like. Thus the image display by a display is improper especiallywhen used for displaying the text documents mainly composed of lettersbecause of low resolution. If it is used for displaying sentences whichcontinue in less than the frame-size volume, it will do. However, if thesentences continue in twice or more times the frame-size volume, theymay be difficult to read and to understand. Due to the relatively lowresolution and the light emission from the display, operations for along time are likely to be hard to eyes.

Electrophoretic display (EPD) and Twist ball-type display (TBD) havebeen proposed as an image display method allowing repetition of theimage display and image erasure. Further displaying method was recentlyproposed, which is disclosed in “Japan Hardcopy '99, the book of thethesis, pp. 249-252”.

In the electrophoretic display method, two substrates including at leastone transparent substrate are opposed together with a spacertherebetween to form a closed space therebetween, and the space isfilled with a display liquid formed of a dispersion medium andelectrophoretic particles, which are dispersed in the dispersion mediumand are different in color from the medium. The image display isperformed by an application of an electrostatic field and in a color ofthe particles or a color of the dispersion medium.

The display liquid is usually formed of isoparaffin-contained dispersionmedium, particles of titanium dioxide or the like, dyes applyingcontrast in color to the particles, and an additive such as a surfaceactive agent, or a charge applying agent.

In the electrophoretic display, the display is performed by utilizingcontrast between particles of a high refractive index (e.g., titaniumdioxide particles) and colored insulating liquid, and therefore theparticles can not hide the colored liquid to a high extent, resulting ina low contrast.

Furthermore, there is a limitation on the kind of dye which is dissolvedin a high concentration in a nonpolar solvent of high resistance whichallows the electrophoresis of particles. A dye showing a white color isnot found. Nor known is a black dye having a high extinctioncoefficient. Therefore the background portion becomes colored so that itis difficult to achieve a good contrast by a white background. Whenwhite particles for formation of images are placed into a coloredliquid, the colored liquid may be moved between the substrate and thelayer of white particles which are moved to the image observation sidesubstrate, or the colored liquid may come into between the whiteparticles, thereby lowering the contrast. The electrophoretic particlescan scarcely uniformly adhere to the image observation side substrate,and thus the resolution is low.

Further, settling and condensation of particles are liable to occur dueto a very large difference in specific gravity between the particles andthe dispersion medium in the display liquid. This is liable to lower thedisplay contrast. Further, it is difficult to display the images withhigh stability for a long time, and remaining of last images is liableto occur. Further, the degree of charging of the particles in the liquidsignificantly changes with time, which also impairs the stability of theimage display.

In the twist ball-type display method, images can be displayed inspecified colors using an image display medium containing numerousmicrocapsules enclosing not only an insulating liquid but also finespheric particle so processed that a half of their surface and the othersurface portion show colors or an optical density which differs fromeach other. Images are displayed in predetermined colors by rotating thefine spheric particles in the microcapsules due to an electric fieldstrength or magnetic strength.

However, according to the twist ball-type display, images are displayedusing fine spherical particles in the insulating liquid within themicrocapsules. This makes it difficult to attain good contrast. Further,the resolution is low since spaces are formed between the microcapsules.In the manufacture of microcapsules, difficulty is entailed in reducingthe size of microcapsules to increase the resolution.

The “Japan Hardcopy '99, the book of the thesis, pp. 249-252” disclosesan image displaying method wherein a closed space is formed by placingtwo substrates as opposed to each other and as spaced from each other,i.e. the two substrates being a laminate of electrodes and a chargetransporting layer, the space being used to enclose the electricallyconductive toner and insulating particles which are different in colorfrom the toner, an electrostatic field being applied to inject chargesinto the electrically conductive toner so that the toner is moved by aCoulomb force applied thereto to display images.

However, the foregoing image displaying method utilizing the chargeinjection phenomenon poses problems. When the electrically conductivetoner carrying the injected charges is moved, insulating particles (e.g.white particles mixed together to form the color of background)interfere with the movement of the toner particles, making theirmovement so difficult that some of them may stop their movement. Thisresults in failure to obtain satisfactory image density and goodcontrast and in reduction of image display rate. To overcome theseproblems, a high voltage drive is necessitated. The resolution isdetermined by the electrodes and is so limited. Furthermore, it isessential to use electrodes, charge-injection layer and electricallyconductive toner, which results in limited manufacture.

SUMMARY OF THE INVENTION

An object of the invention is to provide an image displaying method andan image forming apparatus utilizing a reversible image display medium,which allows repeating of image display and image erasure, and therebycan reduce consumption of image display mediums of paper or the likerelating to the conventional image display and consumable materials suchas developer and ink so that a current demand for reduction inenvironmental loads can be satisfied.

Another object of the invention is to provide an image displaying methodand an image forming apparatus utilizing the reversible image displaymedium, which allow image display in good contrast and high quality.

A further object of the invention is to provide an image displayingmethod and an image forming apparatus utilizing the reversible imagedisplay medium, which allow image display in high resolution and highquality, and more specifically, in high resolution as compared with theelectrophoretic display and the twist ball-type display, and also inhigher resolution when display is performed based on an electrostaticlatent image without employing opposite electrodes.

A still further object of the invention is to provide an imagedisplaying method and an image forming apparatus utilizing thereversible image display medium, which can suppress remaining of lastimage(s), and therefore an image of good quality can be displayed.

An additional object of the invention is to provide an image displayingmethod and an image forming apparatus utilizing the reversible imagedisplay medium, which allow quick image display.

A further object of the invention is to provide an image displayingmethod and an image forming apparatus utilizing the reversible imagedisplay medium, which can reduce a drive voltage required for imagedisplay.

The present invention provides image displaying methods and imageforming apparatuses relating to a reversible image display medium, whichbasically has the following structure.

The reversible image display medium includes:

two substrates opposed to each other with a predetermined gaptherebetween;

one or more developer accommodating cells formed between the twosubstrates, each having a periphery surrounded by a partition wall; and

a dry developer contained in each of the cell(s), the dry developercontaining at least two kinds of frictionally chargeable dry developingparticles having different chargeable polarities and different opticalreflection densities.

The invention provides the following image displaying methods and imageforming apparatuses utilizing the reversible image display medium havingsuch basic structure and having the following features.

(1) Image Displaying Method

(1-1) First Image Displaying Method

This method comprises the steps of: providing a reversible image displaymedium having the foregoing basic structure; and displaying an image bydriving the frictionally charged dry developing particles havingdifferent chargeable polarities in an electrostatic field correspondingto the image to be displayed.

In the image display step, the strength of the electric field to beapplied to the developer is 0.3 V/μm to 3.0 V/μm.

The electric field strength of 0.3 V/μm to 3.0 V/μm is the strength ofthe electric field directly applied to the developer, in other words,applied to the space in the cell accommodating the developer.

(1-2) Second Image Displaying Method

This method comprises the steps of: providing a reversible image displaymedium having the foregoing basic structure; displaying an image byapplying from outside an electrostatic field corresponding to the imageto be formed and an oscillating force to the frictionally charged drydeveloping particles having different chargeable polarities to drive thedeveloping particles; and substantially stopping application of theforegoing oscillating force during the application of the electrostaticfield after image display.

(1-3) Third Image Displaying Method

This method comprises. the steps of: providing a reversible imagedisplay medium having the foregoing basic structure; displaying an imageby applying from outside an electrostatic field corresponding to theimage to be formed to the frictionally charged dry developing particleshaving different chargeable polarities to drive the developingparticles; and charging a surface of the reversible image display mediumon image observation side to carry a potential holding the displayedimage after completion of application of the electrostatic field.

(1-4) Fourth Image Displaying Method

This method comprises the steps of: providing a reversible image displaymedium having the foregoing basic structure; initializing the reversibleimage display medium by stirring the developer in the image displaymedium before displaying an image on the display medium; and displayingthe image by driving the frictionally charged dry developing particleshaving different chargeable polarities within the above-initializedreversible image display medium in an electrostatic field correspondingto the image to be formed.

(1-5) Fifth Image Displaying Method

This method comprises the steps of: providing a reversible image displaymedium having the foregoing basic structure in which at least one kindout of at least two kinds of developing particles are magneticparticles; displaying an image by applying an electrostatic fieldcorresponding to the image to be formed to the frictionally chargeddeveloping particles having different chargeable polarities to drive thedeveloping particles; and applying a stirring force to the developer byaffecting a stirring force on the developer in the reversible imagedisplay medium by a magnetic field from outside before and/or in theimage display step.

If no problem is raised, a combination of at least two features in theforegoing image displaying methods may be employed.

(2) Image Forming Apparatus

(2-1) First Image Forming Apparatus

The first image forming apparatus is one which causes a reversible imagedisplay medium having the foregoing basic structure to display an image.This apparatus comprises a device for initializing the reversible imagedisplay medium by stirring the developer in the image display mediumbefore causing the display medium to display an image; and an imageforming portion for displaying the image by driving the frictionallycharged developing particles having different chargeable polaritieswithin the initialized medium in an electrostatic field corresponding tothe image to be formed.

(2-2) Second Image Forming Apparatus

The second image forming apparatus is one which causes a reversibleimage display medium having the foregoing basic structure to display animage. At least one kind out of at least two kinds of developingparticles in the medium are magnetic particles. This apparatus comprisesan image forming portion for displaying the image by driving thefrictionally charged developing particles having different chargeablepolarities within the reversible image display medium in anelectrostatic field corresponding to the image to be formed; and atleast one device for applying a magnetic stirring force to the developerby affecting the magnetic field on the developer in the reversible imagedisplay medium from outside before and/or in the image display on themedium, thereby applying a stirring force to the developer.

If no problem is raised, a combination of features in the foregoingimage displaying apparatuses may be employed.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example of the reversible imagedisplay medium having opposite electrodes before image display.

FIG. 2 is a sectional view of the medium shown in FIG. 1 on which imagesare displayed.

FIG. 3 is a perspective view showing a first substrate and a grid-likepartition member formed thereon in the medium shown in FIG. 1.

FIG. 4 is a plan view showing the first substrate and independentelectrodes formed thereon in the medium shown in FIG. 1.

FIG. 5 is a view showing an example of the image display on the mediumshown in FIG. 1.

FIG. 6(A) is a sectional view showing another example of the reversibleimage display medium. FIG. 6(B) is a sectional view showing a furtherexample of the reversible image display medium.

FIG. 7(A) and FIG. 7(B) show other example of the reversible imagedisplay medium. FIG. 7(A) is a sectional view of the reversible imagedisplay medium before image display. FIG. 7(B) is a sectional view of anexample of the medium in image display.

FIG. 8(A) and FIG. 8(B) show further example of the reversible imagedisplay medium. FIG. 8(A) is a sectional view of the reversible imagedisplay medium before image display. FIG. 8(B) is a sectional view of anexample of the medium in image display.

FIG. 9 is a plan view showing the medium shown in FIG. 8(A) and FIG.8(B) as partly cut away.

FIG. 10(A) is a sectional view showing another example of the reversibleimage display medium. FIG. 10(B) is a sectional view showing a furtherexample of the reversible image display medium.

FIG. 11 is a view schematically showing an example of an image formingapparatus having an external electrostatic latent image forming device.

FIG. 12(A) and FIG. 12(B) show a schematic view showing the structure ofan example of an image forming apparatus having an ion flow type directelectrostatic latent image forming device.

FIG. 13 is a schematic view showing the structure of an example of animage forming apparatus having a multi-stylus type direct electrostaticlatent image forming device.

FIG. 14 is a schematic view showing the structure of an example of animage forming apparatus having a multi-stylus type direct electrostaticlatent image forming device provided with neighboring controlelectrodes.

FIG. 15 is a view showing a relationship between the strength ofelectric field for driving the developing particles and the contrast.

FIG. 16 is a schematic view showing the structure of a modification ofthe image forming apparatus illustrated in FIG. 11.

FIG. 17 is a view showing an example of a device for charging thesurface of the image display medium after completion of application ofelectrostatic field.

FIG. 18(A) is a schematic view showing the structure of anothermodification of the image forming apparatus of FIG. 11, especially onehaving a device for initializing the medium, and FIG. 18(B) to FIG.18(D) are views showing other examples of the device for initializingthe medium.

FIG. 19 is a sectional view showing another example of the medium ofFIG. 1 while images are displayed.

FIG. 20(A) is a schematic view showing the structure of another exampleof the image forming apparatus having an external electrostatic latentimage forming device. FIG. 20(B) is a view showing part of amodification of the apparatus of FIG. 20(A).

FIGS. 21(A) to FIG. 21(C) are schematic views showing the structures ofother examples of the image forming apparatus.

FIG. 22(A) to FIG. 22(C) are schematic views showing the structures offurther examples of the image forming apparatus.

FIG. 23(A) to FIG. 23(D) are views schematically showing the movement ofmagnetic developing particles when an oscillating magnetic field isapplied with a magnetic field-generating member as opposed to onesurface of the image display medium.

FIG. 24(A) is a view schematically showing the movement of magneticdeveloping particles when an oscillating magnetic field is applied witha magnetic field-generating member as opposed to one surface of theimage display medium. FIG. 24(B) is a view schematically showing themovement of magnetic developing particles when an oscillating magneticfield is applied with magnetic field-generating members as opposed toboth surfaces of the image display medium.

FIG. 25(A) to FIG. 25(C) are views showing the state of images beingdisplayed by applying the oscillating magnetic field to the imagedisplay medium carrying an electrostatic latent image formed over itssurface.

FIG. 26 is a view showing an example of the arrangement of magneticpoles in a magnet plate.

FIG. 27 is a view showing another example of the arrangement of magneticpoles in a magnet plate.

FIG. 28 is a view showing a further example of the arrangement ofmagnetic poles in a magnet plate.

FIG. 29 is a view showing a still further example of the arrangement ofmagnetic poles in a magnet plate.

FIG. 30(A) and FIG. 30(B) are views showing an example of thearrangement of magnetic poles in a rotary magnetic pole roller.

FIG. 31(A) and FIG. 31(B) are views showing other example of thearrangement of magnetic poles in a rotary magnetic pole roller.

FIG. 32(A) to FIG. 32(E) are views showing examples of cellsaccommodating the developer in the reversible image display medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A reversible (in other words, reusable) image display medium accordingto a preferred embodiment of the invention basically has the followingstructure.

The reversible image display medium includes two substrates opposed toeach other with a predetermined gap therebetween; one or more developeraccommodating cells formed between the two substrates, and each having aperiphery surrounded by a partition wall; and a dry developer containedin each of the cell(s). The dry developer contains at least two kinds offrictionally chargeable dry developing particles having differentchargeable polarities and different optical reflection densities.

According to the reversible image display medium, a predeterminedelectrostatic field corresponding to an image to be displayed is formedand is applied to the frictionally charged developing particles in theimage display medium. Thereby, a Coulomb force acting between theelectrostatic field and the charged developing particles can move thedeveloping particles to display the image in predetermined contrast.

After displaying the image, a different electrostatic field, analternating electric field, an oscillating magnetic field (when magneticdeveloping particles are employed) or the like may be formed so that theimage can be erased. Also, the image can be rewritten by forming adifferent electrostatic field. Accordingly, it is not necessary toabandon the image display medium, on which the image is alreadydisplayed. The developing particles are contained in the cell, andtherefore external supply or addition of the developer is not required.Owing to these facts, it is possible to reduce remarkably the use of theimage display medium such as paper sheets as well as consumablematerials such as developer in the prior art. In contrast to the imageformation of the electrophotographic type or the like in the prior art,it is not necessary to melt the toner for fixing it onto a sheet ofpaper or the like, and a majority of the image forming energy, which isrequired in such an image forming apparatus in the prior art, is notrequired.

Owing to the above features, the medium can satisfy a current demand forreduction in environmental loads.

The developer contained in the cell includes at least two kinds ofdeveloping particles having different optical reflective densities, andin other words, exhibiting different contrasts or different colors.Further, the developing particles are dry particles, and one kind of thedeveloping particles can appropriately screen or hide the other kind ofdeveloping particles. Therefore, image display in good contrast can beachieved.

The developer contained in the cell includes at least two kinds of thechargeable dry developing particles, which can be frictionally chargedto have different chargeable polarities. For image display, thedeveloping particles which are mutually reversely charged by thefrictional charging are easily moved by the Coulomb force. This alsoachieves the display in good contrast, and can suppress remaining of thelast image, and also allows quick display of images, and further canreduce a drive voltage required for image display.

The dry developing particles can suppress settling and condensation ascompared with, e.g., electrophoretic particles in a display liquid usedfor electrophoretic image display, because the liquid is not present.This also suppress lowering of the contrast of the image display, andthereby can perform stable image display for a long time. Since thesettling and condensation of the developing particles are suppressed,the remaining of the last image can be suppressed. As compared with theparticles in the liquid, the dry developing particles can perform stableimage display also for the reason that the charging performance thereofchanges with time to a smaller extent.

As compared with the image display by a conventional CRT display or thelike, easy-on-the-eyes image display in high resolution can beperformed.

The electrostatic field corresponding to the image to be formed can beformed, for example, by applying a voltage corresponding to the image tobe formed between electrodes arranged on the substrates of the imagedisplay medium, or by forming an electrostatic latent image on one ofthe substrates.

The electrostatic field can be formed based on the electrostatic latentimage, which is formed on the outer surface of one of the twosubstrates. In this case, the electrostatic field may be formedsimultaneously with formation of the electrostatic latent image, or maybe formed after formation of the electrostatic latent image. Theelectrostatic field may be formed by placing a predetermined potentialon the substrate, which is opposite to the substrate for carrying theelectrostatic latent image. This predetermined potential can be placedby applying the bias voltage to the above opposite substrate, orgrounding the opposite substrate, simultaneously with formation of theelectrostatic latent image, or after formation of the electrostaticlatent image.

Materials useful for substrates and cell partition walls can be selectedfrom a wide range. Useful substrates include, for example, glasssubstrates, hard or soft synthetic resin substrates, and soft filmsubstrates.

At least one of the two substrates forming the medium (arranged on theimage observation side) is light permeable to allow seeing the displayedimage.

In the case of forming an electrostatic latent image. for image displayon the medium surface, the substrate for carrying the electrostaticlatent image can be formed of an insulating substrate. The othersubstrate on the opposite side (e.g., on the non-observation side) maybe an insulating substrate or another kind of substrate. If the othersubstrate is an insulating substrate and ground potential or a biasvoltage must be placed on the other insulating substrate, anelectrically conductive film may be formed on the outer surface of thesubstrate, or the substrate may be entirely made of an electricallyconductive material or a material containing an electrically conductivematerial, although these are not essential. By employing the abovemanner or structure, the substrate can be easily grounded to carry theground potential, or the bias voltage can be easily applied to thesubstrate. An effect of externally shielding the electrical charges bythe substrate on the opposite side can be achieved, if the substrate onthe opposite side is an insulating substrate, and is provided at itsouter surface with the electrically conductive film, or if the substrateitself on the opposite side is the electrically conductive substrate.Thereby, even in the case where the mediums on which images aredisplayed are overlapped together, collapsing of the images can besuppressed, and thereby the images can be stably held.

There is no limitation on the number, size, shape, distribution,arrangement (regular or irregular) and others of thedeveloper-accommodating cells insofar as the image is displayed.Optionally a developer-moving suppressing member and a spacer formaintaining a gap between the substrates may be provided between thesubstrates. The cell partition wall may serve as the developer-movingsuppressing member and the spacer.

FIG. 32(A) to FIG. 32(E) show examples of the shape of cells CEL in thereversible image display medium D. FIG. 32(A) indicates the medium Dhaving a plurality of continuous groove-like cells extending in awidthwise direction of the medium D. FIG. 32(B) shows a plurality ofrectangular cells arranged in series in lengthwise and widthwisedirections of the medium D. In the medium D of FIG. 32(C), the cells arearranged in series in a widthwise direction and displaced in alengthwise direction, and they are arranged in a brick-wall design as awhole. In the medium D of FIG. 32(D), the cells are arranged in seriesin lengthwise and widthwise directions and consist of a plurality ofsquares arranged in a checkerboard design. The medium D of FIG. 32(E)consists of a single cell in which a large number of spacers areprovided between the substrates.

In any case, when small-size cells are used, the deflection of developeris reduced. However, the partition walls occupy a larger portion of thespace between the opposed substrates so that a correspondingly reducedamount of developer is accommodated, resulting in a lower contrast inthe image density. If large cells are used, a good contrast isachievable, but the developer is deflected in a more degree. Accordinglythe size of a cell is determined according to a suppressed degree ofdeflection of developer and a good contrast.

For example, when an electrostatic latent image is formed on thesubstrate, an excessively large gap between the substrates or anexcessively large thickness of each substrate reduces the electric fieldapplied to the developer between the substrates, and therefore impairsthe development performance so that the contrast is lowered. If the gapbetween the substrates is excessively small, this reduces an amount ofthe developer, which can be accommodated in the developer accommodatingcell, so that required contrast can not be achieved. If the thickness ofeach substrate is excessively small, and therefore the whole thicknessof the medium affected by the thickness of each substrate is excessivelysmall, the medium is liable to be curved so that the gap between thesubstrates can not be uniform, and the image irregularities are liableto occur. Accordingly, it is preferable that each substrate has athickness from 5 μm to 100 μm, the gap between the opposite substratesis in a range from 20 μm to 300 μm, and the whole thickness is in arange from 30 μm to 500 μm, although not restricted to these values.

The developing particles may be frictionally charged by applyingmechanical vibrations after accommodating the developing particles inthe cells, or by frictionally charging at least two kinds of developingparticles by stirring and then accommodating the developing particles inthe cells. The latter method is preferable to give the developingparticles frictionally charged in the desired state. At any rate, thedeveloping particles are frictionally charged before image display.

Such reversible image display medium may have or may not haveelectrodes. If the substrate is free of electrode, the medium can be sosimplified and the use of an elastic substrate such as a film is easilyallowed.

Useful reversible image display mediums with electrodes include, forexample, those in which an electrode (preferably transparent electrode)is formed on the internal surface of one of substrates which ispermeable to light while an electrode opposed to the electrode is formedon the internal surface of the other substrate.

The electrode formed on the internal surface of the other substrate mayconsist of a group of independent electrodes formed for respectivepixels.

The image display medium with the electrodes may be provided with leadsfor the electrodes. It is desired that the lead is arranged in thenon-image display region where the partition wall or the like may bepresent.

In either of the reversible image display mediums with and without theelectrode, the developer accommodated in the developer accommodatingcell may contain at least two kinds of dry developing particles, whichhave mutually different chargeable polarities, and different opticalreflective densities (in other words, of different contrasts ordifferent colors). As a typical example, the developer may containpositively chargeable (or negatively chargeable) black particles havinglight absorbing properties and negatively chargeable (or positivelychargeable) white particles having light reflecting properties.

Among at least two kinds of developing particles forming the drydeveloper, at least one kind of the developing particles may benon-conductive particles. In this case, the presence of suchnon-conductive particles allows easy and reliable charging by frictionof the two kinds of developing particles, regardless of whether theimage display medium has the electrodes or not. Thereby, the imagedisplay can be further improved.

Of the two kinds of developing particles forming the dry developer, atleast one kind of the developing particles may be magnetic particles.The existence of such magnetic particles allows affecting a magneticstirring force on the developer (developing particles) by the magneticfield (e.g., oscillating magnetic field) in relation to driving thedeveloping particles in the electrostatic field. Owing to the stirringof the developer, the developing particles can easily move in theelectrostatic field for image display. Thereby, the contrast is furtherimproved and the required voltage for image display can be furtherlowered.

In other words, regardless of whether the image display medium has theelectrodes or not, the existence of such magnetic particles allowsstirring the developer (developing particles) by the magnetic field(e.g., oscillating magnetic field) . Owing to the stirring of thedeveloper, the developing particles can easily move when initializingthe medium or erasing the last image, or displaying the new image in theelectrostatic field for image display. Thereby, the image display isfurther improved.

The developing particles may be stirred by applying AC voltage or likealternating voltage and/or applying mechanical vibrations. Optionallythe stirring may be done using a combination of two or more stirringmeans such as alternating voltage agitation, magnetic agitation,mechanical agitation, ultrasonic wave emission and the like.

One kind of the developing particles may be nonconductive and magneticparticles.

In any one of the foregoing cases, if the developing particles areexcessively small, they have an excessively large adhesiveness, andtherefore cause mutual adhesion of the particles and reduction indeveloping efficiency. Further, such excessively small developingparticles carry a large amount of charges so that a large electric fieldis required for moving the particles for image display, and therefore, ahigh drive voltage is required.

If the developing particles are excessively large, the frictionalcharging can not be performed in an intended manner so that thedeveloping particle moving speed can not be increased sufficiently inthe electrostatic field for image display, and/or good contrast can notbe achieved.

In view of the above as well as the material and others for obtainingthe predetermined characteristics of the developing particles, theappropriate particle diameter(volume average particle diameter) of thenon-conductive developing particle is in a range from 1 μm to 50 μm, andthe appropriate particle diameter(volume average particle diameter) ofthe magnetic developing particle is in a range from 1 μm to 100 μm.

The developing particles can be formed, for example, from a binder resinand a coloring agent, etc. or with a coloring agent alone, etc. Thosewhich are usable are described below.

Binder Resin

The binder resin is not specifically limited in so far as it candisperse a coloring agent, magnetic substance, etc. and is usableusually as a binding agent. Binding resins which are usable forelectrophotography toner are used as a representative example.

Examples of useful binder resins are polystyrene type resins,poly(meth)acrylic type resins, polyolefin type resins, polyamide typeresins, polycarbonate type resins, polyether type resins, polysulfonetype resins, polyester type resins, epoxy resins, urea resins, urethaneresins, fluorine-containing resins, silicone resins and copolymers,block polymers, graft-polymers and polymer blend, etc. of these resins.

The binder resin may have a considerably high glass transitiontemperature (Tg) and need not be a thermoplastic resin.

Coloring Agents

As the coloring agents, the following various kinds of organic orinorganic pigments and dyestuffs having various colors are usable.

Examples of black pigments are carbon black, copper oxide, manganesedioxide, Aniline Black and activated carbon, etc.

Examples of yellow pigments are chrome yellow, zinc yellow, cadmiumyellow, yellow iron oxide, mineral Fast Yellow, Nickel Titanium Yellow,Naphthol Yellow S, Hansa Yellow G, Hansa Yellow lOG, Benzidine Yellow G,Benzidine Yellow GR, Quinoline Yellow Lake, Permanent Yellow NCG andTartrazine Lake, etc.

Examples of orange pigments are red chrome yellow, molybdenum orange,Permanent Orange GTR, Pyrazolone Orange, Vulcan Orange, IndanthreneBrilliant Orange RK, Benzidine Orange G and Indanthrene Brilliant OrangeGK, etc.

Examples of red pigments are red iron oxide, cadmium red, red lead,mercury sulfide, Permanent Red 4R, Lithol Red, Pyrazolone Red, WatchungRed, Lake Red D, Brilliant Carmine 6B, eosine lake, Rhodamine Lake B,alizarin lake and Brilliant Carmine 3B, etc.

Examples of violet pigments are manganese violet, Fast Violet B andMethyl Violet Lake, etc.

Examples of blue pigments are prussian blue, cobalt blue, Alkali BlueLake, Victoria Blue Lake, Phthalocyanine Blue, Phthalocyanine Bluecontaining no metal, partially chlorinated Phthalocyanine Blue, Fast SkyBlue and Indanthrene Blue BC, etc.

Examples of green pigments are chrome green, chromium oxide, PigmentGreen B, Malachite Green Lake and Final Yellow Green G, etc.

Examples of white pigments are zinc white, titanium oxide, antimonywhite and zinc sulfide, etc.

Examples of extender pigments are barite powder, barium carbonate, clay,silica, white carbon, talc and alumina white, etc.

Examples of various kinds of dyestuffs such as basic, acid, disperse andsubstantive dye are Nigrosine, Methylene Blue, Rose Bengale, QuinolineYellow and Ultramarine Blue, etc.

These coloring agents are usable alone or in combination of plural ofthem.

Especially in white-black display, carbon black is preferable as a blackcoloring agent and titanium dioxide as a white coloring agent.

Especially in the case of preparing developing particles from a mixtureof a white pigment and a meltable binding resin(binder resin), it ispreferable to use the white pigment in an amount of at least 10 parts byweight, more preferably at least 20 parts by weight, per 100 parts byweight of raw monomer of white particles, in order to obtain sufficientwhiteness. It is desirable to use the white pigment in an amount of upto 60 parts by weight, more preferably up to 50 parts by weight, inorder to secure sufficient dispersibility of the white pigment. Over 60parts by weight of the white pigment, the binding of the pigment and thebinding resin will decrease and the dispersion of the pigment willdeteriorate. Less than 10 parts by weight of the white pigment, thedeveloping particles having a different color will not sufficiently beshaded by the pigment.

Although carbon black is preferable as the black coloring agent, it ispossible to use magnetic particles or magnetic fine powder such asmagnetite, ferrite, etc. as the coloring agent in order to providemagnetic character to the developing particles.

Other Additives

Examples of additives preferably usable other than the above binderresin or coloring agent are magnetic substance, charge-controllingagent, resistance adjusting agent, etc.

Charge-Controlling Agent

The charge-controlling agent is not specifically limited in so far as itprovides a charge to the developing particles by friction-charging.

Examples of plus-charge-controlling agents are Nigrosine dye,triphenylmethane compound, quaternary ammonium salt compound, polyamineresin, imidazole derivative, etc.

Examples of minus-charge-controlling agents are salicylic acid-metalcomplex, metal-containing azo dye, metal-containing oil-soluble dye(including metal ion or metal atom), quaternary ammonium salt compound,calixarene compound, boron-containing compound (benzilic acid-boroncomplex), nitroimidazole derivative, etc.

Other than the above, as charge-controlling agents are usable metaloxides such as ultrafine silica particles, ultrafine titanium oxideparticles, ultrafine alumina particles, etc., nitrogen-containing cycliccompounds such as pyridine or its derivative, salt, various organicpigments, resins containing fluorine, chlorine, nitrogen, etc.

Magnetic Substances

Magnetic particles and magnetic fine powder are usable. Examples ofthese substances are ferromagnetic elements, alloy or compoundscontaining the element. Examples thereof are those containing aconventionally known magnetic substance such as magnetite, hematite,ferrite or like alloys or compounds of iron, cobalt, nickel, manganese,etc., other ferromagnetic alloy, etc. The magnetic powder may havevarious shapes such as particle, needle, thin flat shape, etc. and issuitably usable.

Resistance Adjusting Agent

Resistance adjusting agents include similar compounds to the abovemagnetic powder and coloring agent.

Examples of resistance adjusting agents are metal oxides, graphite,carbon black, etc. having various shapes such as thin flat, fibrous orpowder shape, etc.

Below is explained an example of preparing developing particles.

Prescribed amount of each of components selected from the above binderresin, magnetic powder, coloring agent, charge-controlling agent,resistance adjusting agent and other additives is prepared, and thosecomponents are mixed thoroughly. The mixture is further mixed withheating by use of press-kneader, twin-screw mixing device, etc. Aftercooling, the mixture is roughly pulverized with use of hammer mill,cutter mill, etc. and then finely pulverized with use of jet mill,angmill, etc. The resulting powder is classified by a wind classifier,etc. to a predetermined average particle size to obtain developingparticles.

A developer having a predetermined amount of charges is obtained bymixing and stirring thus obtained particles having different chargeablepolarities and contrasts(optical reflective densities) at apredetermined rate thereof. A third agent such as fluidization agent maybe added thereto to improve fluidity of the developer.

Fluidization Agent

Examples of fluidity improving agents are silica, alumina, titaniumoxide, barium titanate, magnesium titanate, calcium titanate, strontiumtitanate, zinc oxide, siliceous sand, clay, mica, wallastonite,diatomaceous earth, chromium oxide, cerium oxide, red iron oxide,antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate,barium carbonate, calcium carbonate, silicon carbide, silicon nitride,etc.

Particularly preferable are fine powder of silica, aluminum oxide,titanium dioxide and magnesium fluoride. The fluidity improving agent isused either alone or in combination.

In the image display employing the reversible image display medium, theelectrostatic field to be applied to the developing particles can beformed, for example, based on the electrostatic latent image, which isformed on, or brought close to or into contact with, e.g., the surfaceof one (e.g., on the image observation side) of the two substrates inaccordance with the image to be displayed. The formation of theelectrostatic field may be performed simultaneously with or after theformation or approach of the electrostatic latent image. The formationof the electrostatic field is performed, e.g., by placing apredetermined potential, which is required for forming the electrostaticfield, on the substrate opposite to the substrate, on which theelectrostatic latent image is to be formed. The above predeterminedpotential can be placed by applying a bias to the opposite substrate, orby grounding the opposite substrate.

The electrostatic latent image may be formed directly on the mediumsurface (substrate surface), e.g., by a device for directly forming theelectrostatic latent image, or may be formed by transferring theelectrostatic latent image, which is formed outside the medium by anexternal electrostatic latent image forming device, onto the mediumsurface (substrate surface). The electrostatic latent image, which isformed outside the medium by an external electrostatic latent imageforming device, may be brought close to or into contact with the mediumsurface (substrate surface).

The direct electrostatic latent image forming device may be of variousdischarging types, in which the electrostatic latent image charges areplaced by performing the discharge to the medium surface in accordancewith the image to be displayed, or of various charge injection types, inwhich the electrostatic latent image charges are placed by injectingcharges to the medium surface in accordance with the image to bedisplayed. For example, the devices of the former type may be of an ionflow type, and also may be of a multi-stylus type having anelectrostatic record head, in which recording electrodes are arranged ina predetermined direction (e.g., main scanning direction for substratescanning by the device). In an example of the latter type, the device ofthe multi-stylus type may be used, which includes an electrostaticrecord head, in which the recording electrodes are arranged in apredetermined direction (e.g., main scanning direction for substratescanning by the device), and neighboring control electrodes are arrangedclose to the recording electrodes.

The external electrostatic latent image forming device may be configuredsuch that the electrostatic latent image corresponding to the image tobe displayed is formed on the electrostatic latent image carrier, andthen is transferred onto, or brought close to or into contact with thesubstrate surface. More specifically, the electrostatic latent imagecorresponding to the image to be displayed may be formed, e. g., on aphotoconductive member such as a photosensitive member, and may betransferred onto, or brought close to or into contact with the substratesurface. Alternatively, the electrostatic latent image corresponding tothe image to be displayed may be formed on a dielectric member, and maybe transferred onto, or brought close to or into contact with thesubstrate surface.

The image display may be performed with the electric field formingdevice including one of the foregoing electrostatic latent image formingdevices.

By forming the electrostatic latent image on the image display medium inthe foregoing transfer manner or the direct formation manner or bybringing the electrostatic latent image close to or into contact withthe image display medium, the image holding properties of the medium canbe improved. In particular, the image holding properties can be improvedin the case of using developer having high flowability or developerhaving flowability which can be increased by the developer stirringoperation prior to the image display.

In the reversible image display medium having the opposite electrodes,the electrostatic field for the image display can be formed by applyinga voltage across the opposite electrodes. The electrostatic fieldformation device for such medium will be described later.

In the reversible image display medium without an electrode or with anelectrode on only one of the substrates, the electrostatic field to beapplied to the developing particles can be formed, for example, byarranging an electrode or electrodes on the outer surface(s) of themedium and applying a voltage through the electrode(s).

In both the reversible image display mediums with and without theelectrode, image erasing processing (initializing processing) may beperformed for erasing the previously displayed image prior to the newimage display.

The image erasing processing (initialization processing) can beperformed, e.g., by forming an electric field, which can move thedeveloping particles forming the developer in the image display medium,and/or applying a stirring force to the developer. The application ofthe stirring force can be performed, e.g., by forming an alternatingelectric field, forming an oscillating magnetic field, emittingultrasonic waves, and/or applying mechanical vibrations.

For the image display, therefore, various kinds of image easing devices(medium initializing devices) can be appropriately employed. Such imageerasing devices may include the electric field forming device forforming the electric field moving the developing particles, the stirringdevice for applying a stirring force to the developing particles, orboth the electric field forming device and the stirring device.

For example, under the electric field, one kind of the developingparticles, which have the same optical reflection density (i.e., thesame contrast or the same color), among the two kinds of developingparticles described above may be collected to one of the substrates, andthe other kind of developing particles having the same opticalreflection density may be collected to the other substrate. Thereby, theimage erasure can be performed. Further, the next image formation can beperformed by moving the developing particles on only the image portionso that the image display can be performed smoothly and reliably in highquality.

For example, in the operation of stirring the developer (developingparticles), the image is erased, and the amount of charges and theflowability of the developing particles are improved. Thereby, the nextimage formation can be performed smoothly and reliably in a highquality.

In addition, for example, when the developer (developing particles) isstirred before image display, the image is erased, remaining of imagesis suppressed, the developing particles are prevented from locallyexisting in the medium (cells), are uniformly distributed therein andare brought into properly charged state. Thereby high-quality images aredisplayed in forming the images.

The electric field forming device for image erasing (initialization) mayinclude a pair of electrodes (usually made of metal) or dielectricmembers, which are arranged on the opposite sides of the reversibleimage display medium, and a power supply device for applying a biasvoltage across these electrodes or dielectric members.

In addition to the above, it is possible to employ various kinds ofelectric field forming devices of the discharging type, in which theelectric field is formed by performing the discharging to the imagedisplay medium, and various kinds of electric field forming devices ofthe charge injection type, in which the electric field is formed byinjecting the electric charges to the reversible image display medium.The devices of the former type may be specifically are a Corona chargingdevice, an electric field forming device of an ion flow type, and anelectric field forming device of the multi-stylus type having a head, inwhich electrodes are arranged in a predetermined direction. The deviceof a latter type may be specifically an electric field forming device ofthe multi-stylus type, in which electrodes are arranged in apredetermined direction, and neighboring control electrodes are arrangedclose to the electrodes.

The stirring device may be configured as follows:

Thus, the stirring device may be configured to form an alternatingelectric field applied to the reversible image display medium.

This device can be utilized if at least one kind of developing particleshave the electrically insulating property.

Also, the stirring device may be configured to form an oscillatingmagnetic field applied to the reversible image display medium.

This device can be utilized if at least one kind of developing particlescontain a magnetic material.

Further, the stirring device may be configured to emit ultrasonic wavesto the reversible image display medium.

The stirring device may be configured to apply mechanical vibrations tothe reversible image display medium.

The stirring device may be formed of a combination of the foregoing twoor more structures.

The alternating electric field applying device and the oscillatingmagnetic field applying device can efficiently stir the developer.

As already described, the stirring of the developer (developingparticles) improves the amount of charges and the flowability of thedeveloping particles, and thereby can achieve smooth and reliable imagedisplay with high quality.

By stirring the developer prior to the image display, the amount ofcharges of the developing particles is stabilized. This likewiseachieves good image display. Further, the allowable ranges of thechargeability and flowability of the developer can be widened.

For the image display using the reversible image display medium eitherwith or without the electrode, the developer may be stirred also for thepurpose of performing the foregoing image erasing processing, orindependently of the image erasing processing.

When using the image display medium without an electrode or with anelectrode on only one of the substrates, for example, the electrostaticlatent image corresponding to the image to be displayed may be formed,e.g., on the surface (substrate surface) of the image display medium,and the electrostatic field may be formed based on the electrostaticlatent image simultaneously with or after the formation of theelectrostatic latent image, and the developer may be stirred, forexample, simultaneously with and/or before formation of theelectrostatic field.

For the image display medium provided with the opposite electrodes, avoltage may be applied across the opposite electrodes to form theelectrostatic field, and, for example, the developer may be stirredbefore or simultaneously with the formation of the electrostatic field.

Regardless of whether the electrode is employed or not, the developercan be stirred, e.g., by a stirring device, which is opposed to an imagedisplay medium transporting path, and is located in or upstream to theregion for forming the electrostatic field by the electric fieldformation device in the relative transporting direction of the imagedisplay medium with respect to the electric field formation device.

The developer stirring device and method may be the same as or similarto those already exemplified in connection with the image erasingprocessing.

By stirring the developer for the image display, the contrast can befurther improved, and the drive voltage can be further lowered.

For the image display employing the reversible image display medium, theelectrostatic latent image may be formed on the surface (substratesurface) of the image display medium in such a manner that the mediumsurface is uniformly charged to carry the predetermined potential beforeformation of the electrostatic latent image, and the electrostaticlatent image in accordance with the image to be displayed is formed onthe charged region. Based on the electrostatic latent image, thepredetermined electrostatic field is formed in accordance with the imageto be displayed. Thereby, the developing particles may be moved for theimage display.

The formation of electrostatic latent image on the medium can beperformed, e.g., by directly forming it on the medium surface charged inthe charging step, or by transferring the electrostatic latent imageformed on the electrostatic latent image carrier outside the medium ontothe medium surface charged in the charging step.

The region of the electrostatic latent image formed on the medium mayhave such charging characteristics that the region is charged to carrythe same polarity as or the polarity different from the charged polarityof the region of the medium surface, which is uniformly charged prior tothe electrostatic latent image formation, or that the region of thelatent image is charged to 0 V.

According to the above manner, in which the electrostatic latent imageis written onto the charged region formed by uniformly charging thesurface of the image display medium to carry the uniform potential, thecharged developing particles in the developer accommodating cell(s) canbe moved. Further, such an electrostatic field, which is enough to holdthe moved developing particles is formed. In other words, afteruniformly charging the surface of the image display medium to carry thepredetermined potential, the electrostatic latent image is written ontothe charged region, whereby the image holding properties are improved.Particularly, in the case of using the developer having high flowabilityor the developer having the flowability which can be increased by thedeveloper stirring operation prior to the image display, the advantagesrelating to the image holding can be achieved. Owing to the above,images of good contrast and high quality can be stably displayed for along time.

According to the various reversible image display mediums describedabove, the images of good contrast, high resolution and high quality canbe stably displayed for a long time. Further, remaining of last imagescan be suppressed, and therefore good reversibility can be achieved.These improve the quality of the displayed image. The image display canbe quickly performed with lower drive voltage. The image display can beperformed with fewer irregularities.

If an electric field having an excessively low electric field strengthis applied to the developer (developing particles) in image display(image display step) on the reversible image display medium having theforegoing basic structure, a low electrostatic force is applied to thedeveloping particles, so that the particles can not move with ease,resulting in low contrast and low density reproducibility. If anelectric field having an excessively high electric field strength isapplied, the particles charged to a polarity which should be moved in apredetermined direction according to the electric field are moved in theforegoing direction together with the charged particles having reversedpolarity which should be moved in the opposite direction, resulting inreduced contrast, impaired density reproducibility and lowered densityuniformity.

In image display (in image display step), if the strength of electricfield applied to the developer (electric field for driving thedeveloping particles) is in the range of 0.3 V/μm to 3.0 V/μm, thedeveloping particles can smoothly and properly move and images can bedisplayed in good contrast, and with high image density (densityreproducibility). The electric field strength of 0.3 V/μm to 3.0 V/μm isapplied directly to the developer.

Of the two kinds of developing particles forming the dry developer inthe reversible image display medium, at least one kind of the developingparticles may be magnetic particles. Such magnetic developing particlesare usually prepared by dispersing magnetic powder in the particles. Inthis case, the particles are lowered in electrical resistance due to themagnetic powder, and may be abnormally charged by injection of chargebut can be prevented from being so charged by applying the electricfield having the electric field strength of 3.0 V/μm or less to thedeveloping particles for driving the developing particles.

When an image is displayed by the reversible image display medium havingsaid structure, an oscillating force may be applied from outside to thedeveloping particles in the medium as described above. The stirringforce may be applied to the developing particles of the medium indriving the developing particles by an electrostatic field. Thereby thedeveloping particles can be smoothly moved, and contrast can be moreimproved and a lower voltage drive can be allowed.

When the application of the oscillating force is substantially stoppedduring application of electrostatic field after image display, the imagedisplay can be smoothly performed and the displayed image is suppressedfrom being disturbed by external oscillating force so that the displayedimage can be stably held.

If at least one kind out of two kinds of frictionally chargeabledeveloping particles forming the dry developer and having differentchargeable polarities and different optically reflection densities aremagnetic particles, the application of oscillating force to thedeveloping particles in image display step can be performed by applyingan oscillating magnetic field. The substantial stop of application ofoscillating force in the step of substantial stop of application ofoscillating force can be done by substantial stop of application ofoscillating magnetic field during the application of electrostatic fieldafter image display. If the oscillating magnetic force is applied to thedeveloping particles in the medium in image display, a magnetic stirringforce can be applied to the developing particles of the medium indriving the developing particles by an electrostatic field. Thereby thedeveloping particles can be smoothly moved in the electrostatic field,and contrast can be improved and a lower voltage drive is allowed.

In the case of substantial stop of application of oscillating magneticfield during the application of electrostatic field after image display,the term “stop of application of oscillating magnetic field” refers toboth cases of a) stop (exclusion or removal) of application ofoscillating magnetic field and static magnetic field and b) stop(exclusion or removal) of application of oscillating magnetic fieldwhile allowing remaining of static magnetic field. The term “substantialstop” of application of oscillating magnetic field includes the case ofcomplete stop thereof (complete exclusion or removal) and the case wherethe application of oscillating magnetic field still remains but thedisplayed image can not be disturbed any more by the oscillatingmagnetic field.

The application of external oscillating force includes, for example,application of oscillating magnetic field, application of mechanicaloscillating force, application of alternating electric field (e.g. ACelectric field), emitting ultrasonic waves, etc.

In the case where the oscillating force is applied from outside in imagedisplay, substantial stop of application of oscillating force isconducted, whatever the application of oscillating force from outsidemay be, for example, in order to assure the retention of displayed imagewhile the electrostatic field having a strength of at least 0.5 V/μm isapplied from outside to the developer (developing particles) after imagedisplay.

After completion of application of electrostatic field, the surface ofthe reversible image display medium on the image observation side may becharged to a potential holding the displayed image. Thereby thedisplayed image can be more stably held.

The term “a potential holding the displayed image” used herein means apotential corresponding to one of the charged polarities of thefrictionally charged developing particles having different polarities.Slight repulsive force is exerted on the developing particles of thesame polarity as the charged potential among the developing particleslocally existing toward the substrate on the image observation side,while the charged potential positively attracts the developing particlesof reversed polarity toward the substrate on the image observation sidein preference. Thus it is the potential for retaining the displayedimage as a whole. If such potential for retaining the displayed image istoo high, it results in disturbance of images. To avert this problem, aproper potential is, e.g., 100 V or less in terms of absolute value. Alower limit of the potential for retaining the displayed image is suchthat at least the displayed image can be retained. For example, it isabout 10 V in terms of absolute value.

The magnetic developing particles are prepared usually using a magneticpowder, and in most cases, developing particles of deep color such asblack are produced. When the magnetic developing particles include thoselocally existing at the substrate on the image observation side in imagedisplay, the locally existing magnetic developing particles can beeasily seen through the substrate on the image observation side, even ifthey are slightly separated from the substrate due to the repulsiveforce. On the other hand, it is difficult to see the developingparticles, if remote from the substrate, which are frictionally chargedto a polarity opposite to that of magnetic developing particles, thesecharged developing particles being low in optically reflection density(such as white color) and existing locally toward the substrate on theimage observation side in image display.

Therefore, if the magnetic developing particles are used, and when thesurface of reversible image display medium is charged to carry apotential holding the displayed image after application of electrostaticfield, the charged polarity of the charged potential may correspond tothe charged polarity of the magnetic developing particles.

The surface of reversible image display medium may be charged to carry apotential holding the displayed image after application of electrostaticfield, irrespectively of whether or not the developer present in themedium contains the magnetic developing particles. Thereby the displayedimage can be more stably held.

In the reversible image display medium having the foregoing basicstructure, the developer in the image display medium may be stirredbefore image display to initialize the medium, and an image can bedisplayed on the initialized image display medium as described above.

The image display medium may be initialized as stated above before imagedisplay, whereby the displayed image can be erased and remaining of lastimages can be suppressed so that a new image can be displayed in higherquality.

Preferred examples of the initializing processing include application ofalternating electric field to the developer in the reversible imagedisplay medium. Accordingly preferred examples of the initializingdevice include initializing devices in which the developer is stirred byapplication of alternating electric field to the developer in thereversible image display medium.

The developer can be efficiently stirred by application of alternatingelectric field.

The wave shape of alternating electric field such as sine wave,rectangular wave and the like which are effective in stirring thedeveloper can be employed.

When images are displayed by forming an electrostatic latent image onthe surface of the reversible image display medium, a remainingelectrostatic latent image can be erased by application of alternatingelectric field before image display, whereby remaining of images can besuppressed and images of higher quality can be formed.

In the case of application of alternating electric field to thedeveloper for initialization or in the case of using a device forapplication of alternating electric field to the developer as aninitializing device in the image forming apparatus, the strength of thealternating electric field to be applied to the developer or to thespace in the cells accommodating the developer in the image displaymedium is, for instance, 0.5 V/μm or higher.

If the strength of electric field to be applied is less than 0.5 V/μm,the developer can not be sufficiently stirred.

The upper limit of the strength of electric field to be applied is, forexample, preferably approximately less than 2.0 V/μm, although notlimited thereto, to prevent the undesired charging.

A preferred frequency of alternating electric field to be applied is,for example, 5 kHz or less.

If the frequency of alternating electric field to be applied is greaterthan 5 kHz, the developing particles can not easily move, whereby theycan not be sufficiently stirred. If the frequency of alternatingelectric field to be applied is excessively low, it takes a longer timeto stir the developer sufficiently.

The alternating electric field is applied, for example, such that (thefrequency of alternating electric field)×(the time for application ofalternating electric field)=20 or more.

The equation of (the frequency of alternating electric field [Hz])×(thetime (sec) for application of alternating electric field) represents thevalue corresponding to the number of vibrations of developer, in otherwords, the value showing the extent of stirring the developer. If thisvalue is less than 20, the developer can not be sufficiently stirred. Inthe reversible image display medium having the foregoing basicstructure, at least one kind out of two kinds of developing particlesmay be magnetic particles. When the reversible image display mediumcontaining the magnetic particles is used, a stirring force may beapplied to the developer by affecting a stirring force on the developerby a magnetic field from outside before and/or in image display step.

When a magnetic stirring force is applied to the developer before imagedisplay, the developer containing the magnetic developing particles isstirred before image display, whereby the remaining images are erased,and remaining of images can be suppressed. Further uneven distributionof developing particles in the medium is prevented, the uniformity ofdistribution is achieved, and the developing particles are charged in aproper state. Thereby image display of high quality can be done in imagedisplay step. When a magnetic stirring force is applied to the developerin image display step, the developing particles to be moved byapplication of an electric field are smoothly moved and high-qualityimages can be displayed more rapidly with lower voltage drive.

The image forming apparatus may be provided with at least one device forapplying a magnetic stirring force to the developer by affecting astirring force on the developer from outside by the magnetic fieldbefore and/or when an image is displayed on the reversible imagedisplay.

When the device for applying a magnetic stirring force affects astirring force on the developer before image display, the developercontaining the magnetic developing particles can be stirred by thedevice before image display, whereby the last images are erased, andremaining of the last images can be suppressed. Further unevendistribution of developing particles in the medium is prevented, theuniformity of distribution is achieved, and the developing particles arecharged in a proper state. Thereby image display of high quality can beachieved.

When a magnetic stirring force is applied to the developer by the devicein image display, the device affects a stirring force on the developingparticles to be moved by application of electrostatic field, wherebythey are smoothly moved and high-quality images can be displayed quicklywith lower voltage drive.

When such devices for use before image display and for use in imagedisplay, respectively are provided, images of higher quality can beformed.

If the developer is stirred by a magnetic force, the developer can beefficiently stirred.

The method of applying a magnetic stirring force comprises, for example,placing a magnetic field-generating member as opposed to the reversibleimage display medium, and relatively moving the medium and the surfaceof the magnetic field-generating member to oscillate the magnetic fieldstrength to be applied to the developer or in other words to generate anoscillating magnetic field for application of a stirring force.

In this case, the magnetic field-generating member may be opposed to atleast one surface of the reversible image display medium.

At any rate, the following cases can be exemplified concerning at leastone magnetic field-generating member.

-   (a) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member to    be used is one in which magnetic poles are arranged in said    predetermined direction.-   (b) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction and a direction across the predetermined    direction, and the magnetic field-generating member to be used is    one in which magnetic poles are arranged in the direction across the    predetermined direction.-   (c) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member to    be used is one wherein magnetic poles are arranged in a direction at    a specified angle to the predetermined direction.-   (d) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member to    be used is one wherein at least two rows of magnetic poles are    arranged in a direction across said predetermined direction, and    wherein in adjacent rows of magnetic poles, the positions of N and S    magnetic poles are displaced from each other.

In any case, the magnetic field-generating member may be opposed to eachside of image display medium. In this case, magnetic field-generatingmembers opposed to both sides of the medium having differentarrangements of magnetic poles may be used.

In any case, the shape of the magnetic field-generating members can beany of a rotatable roller shape, a plate shape, and the like.

The device for applying a magnetic stirring force is, for example, onewhich includes the magnetic field-generating member as opposed to thereversible image display medium, and which is capable of oscillating themagnetic field strength to be applied to the developer by relativemovement of the reversible image display medium and the surface of themagnetic field-generating member (in other words, capable of generatingan oscillating magnetic field).

In this case, the device for applying a magnetic stirring force may beprovided with the magnetic field-generating member opposed to at leastone surface of the reversible image display medium. At any rate, thefollowing cases may be exemplified concerning at least one magneticfield-generating member in at least one device for applying a magneticstirring force.

-   (a) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member    has magnetic poles arranged in said predetermined direction.-   (b) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction and a direction across the predetermined    direction, and the magnetic field-generating member has magnetic    poles arranged in the direction across the predetermined direction.-   (c) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member    has magnetic poles arranged in a direction at a specified angle to    the predetermined direction.-   (d) The surface of the magnetic field-generating member and the    reversible image display medium are relatively moved in one    predetermined direction, and the magnetic field-generating member    has at least two rows of magnetic poles arranged in a direction    across said predetermined direction, wherein in adjacent rows of    magnetic poles, the positions of N and S magnetic poles are    displaced from each other.

In any case, at least one device for applying a magnetic stirring forcemay have magnetic field-generating members opposed to both sides of theimage display medium. In this case, the opposed magneticfield-generating members having different arrangements of magnetic polesmay be used.

In the image forming apparatus, the shape of the magneticfield-generating members can be any of a rotatable roller shape, a plateshape, and the like.

Specific examples of the reversible image display medium, image displaymethod and image forming apparatus will now be described below withreference to the drawings.

<Reversible Image Display Medium>

Reversible Image Display Medium 11

FIGS. 1 and 2 show an example of the reversible image display medium. Amedium 11 shown in FIGS. 1 and 2 includes first and second substrates111 and 112. These substrates 111 and 112 are opposed to each other witha predetermined gap therebetween. A partition member 113 is arrangedbetween the substrates 111 and 112 for keeping a predetermined gapbetween the substrates. The partition member 113 serves also as a spacerbetween the substrates 111 and 112. The partition member 113 couples andfixes the substrates 111 and 112 together.

The first substrate 111 is formed of a light-transparent plate such as aglass plate, a transparent resin film or the like. The substrate 111 islocated on the image observation side.

The partition member 113 is also a group of partition walls formingdeveloper accommodating cells (see FIG. 3). The partition member 113 isarranged on the inner surface of the first substrate 111 and has agrid-like form as shown in FIG. 3. Thereby, the partition member 113defines a plurality of developer accommodating cells 116, each of whichhas a square form and is surrounded by a portion of the partition member113. The partition wall has a width(thickness) a and a height h, and isspaced by a distance of pt from the neighboring wall.

A first electrode 114 is a transparent electrode, and formed on theinner surface of the first substrate 111 opposed to the second substrate112. The first electrode 114 includes a plurality of independentelectrodes 114 a arranged in a grid-like form. Each of the independentelectrodes 114 a is transparent and made of, e.g., indium tin oxide(ITO) The independent electrodes 114 a are arranged in cells 116,respectively, with a distance between the neighboring independentelectrodes, which is substantially identical with the partition wallthickness α. Thus one cell corresponds to one pixel.

The second substrate 112 is not essentially required to be transparent,but is formed of a light-transparent plate such as a glass plate, aresin film or the like in this embodiment.

The second substrate 112 is provided at its inner surface opposed to thefirst substrate 111 with a second electrode 115. The second electrode115 in this example extends continuously throughout an image displayregion of the inner surface of the substrate. The second electrode 115is not essentially required to be transparent, but may be formed of,e.g., transparent ITO.

Each cell accommodates the dry developer DL including the white andblack developing particles WP and BP, which are mutually andfrictionally charged.

Each cell is sealed so that the developer DL does not leak from thecell.

The independent electrode 114 a forming the second electrode 114 in theimage display medium 11 is connected to or provided with a lead portion110 as shown in FIG. 4, and is connected to an electrode select circuit117 through the lead portion as shown in FIG. 1. The electrode selectcircuit 117 is connected to a positive drive voltage generating circuit118 a, a negative drive voltage generating circuit 118 b and a displaydata control portion 119. Each independent electrode 114 a isindependently supplied with a drive voltage from the electrode selectcircuit 117. The display data control portion 119 receives display datafrom display data output means (not shown) such as a computer, a wordprocessor, a facsimile machine or the like, and controls the electrodeselect circuit 117 based on the supplied data. In other words, theseelectrode select circuit and others form an example of the electricfield forming device or the image forming apparatus for the reversibleimage display medium provided with the electrodes.

For example, the second electrode 115 in the image display medium 11 isgrounded, or a bias voltage is applied from a bias source (not shown) tothe electrode 115, if necessary, and the positive or negative drivevoltage generating circuit 118 a or 118 b applies the predeterminedvoltage across the electrode 115 and each independent electrode 114 avia the electrode select circuit 117, which is controlled to perform thedesired image display by the display data control portion 119. Thereby,the predetermined electric field is formed for each pixel so that thedeveloping particles WP and BP, which are mixed in the developer DL asshown in FIG. 1, move in accordance with the respective electric fieldsas shown in FIG. 2. In this manner, the image can be displayed inpredetermined contrast. For example, image display can be performed asshown in FIG. 5. In FIG. 5, Bk indicates a portion displayed in black, Windicates a portion displayed in white.

A roller R2 shown with a chain line in FIG. 2 will be described later.

Reversible image Display Mediums 12, 12′

FIG. 6(A) and FIG. 6(B) show other examples of the reversible imagedisplay medium.

The reversible image display medium 12 shown in FIG. 6(A) is identicalwith the medium 11 of FIG. 1 if the medium 11 is such that at least thefirst substrate 111 is formed of a material having a light transmittingproperty and an insulating property and that the independent electrodes114 a are not provided.

The medium 12 is identical in other respects with the medium 11 ofFIG. 1. In FIG. 6(A), the same components and parts as in the medium 11are indicated with the same reference symbols.

The reversible image display medium 12′ shown in FIG. 6(B) is identicalwith the medium 11 of FIG. 1 if the medium 11 is such that at least thesecond substrate 112 is formed of a material having a light transmittingproperty and an insulating property and that the electrode 115 is notprovided. The medium 12′ has the substrate 112 on the image observationside.

The medium 12′ is identical in other respects with the medium 11 ofFIG. 1. In FIG. 6(B), the same components and parts as the medium 11 areindicated with the same reference symbols.

In the medium 12 (or the medium 12′), for example, the electrode 115 ofthe second substrate 112 (electrodes 114 a in the medium 12′) is (are)grounded. In addition, for example, over the external surface of thefirst substrate 111 (second substrate 112 in the medium 12′), a) anelectrode or electrodes are provided to selectively apply a voltagecorresponding to the image to be formed between the electrode(s) and theground electrode(s), b) an electrostatic latent image corresponding tothe image to be formed is directly formed, or c) an image carriercarrying the electrostatic latent image corresponding to the image to beformed is contacted (or made to come close), whereby the electric fieldfor driving the developing particles is applied to the developer DL,whereby an image can be displayed.

The electrode 115 of the medium 12 and the electrodes 114 a of themedium 12′ may be electrode(s) having an intermediate resistance value.

Reversible Image Display Medium 13

FIG. 7(A) shows another example of the reversible image display medium.

The reversible image display medium 13 shown in FIG. 7(A) is identicalwith the medium 11 of FIG. 1 if the medium 11 is such that at least thefirst substrate 111 is formed of a material having a light transmittingproperty and an insulating property and that the first substrateelectrode 114 and the second substrate electrode 115 are not provided.

The medium 13 is identical in other respects with the medium 11 ofFIG. 1. In FIG. 7(A), the same components and parts as in the medium 11are indicated with the same reference symbols.

Reversible Image Display Medium 14

FIG. 8(A) shows a further example of the reversible image displaymedium.

The reversible image display medium 14 shown in FIG. 8(A) is identicalwith the medium 11 of FIG. 1 if the medium 11 is such that at least thefirst substrate 111 is formed of a material having a light transmittingproperty and an insulating property, that the first substrate electrode114 and the second substrate electrode 115 are not provided, and that apartition member 113 is provided which consists of a plurality ofpartition walls 113 a extending in parallel with the lengthwise side ofthe medium 14 instead of the grid-like partition member (see FIG. 9).The developer-accommodating cell 116 is provided between the neighboringpartition walls 113 a. Each cell 116 accommodates the developer DLcontaining mutually frictionally charged white developing particles WPand black developing particles BP.

In the periphery of the medium 14, the two substrates 111, 112 areheat-sealed to form a sealing portion 140. The part 140 a of the sealingportion 140 is provided at the both ends of the longitudinal partitionwalls 113 a extending in the lengthwise direction, and serves also asthe partition wall forming the cell 116.

As shown in FIG. 9, the partition walls 113 a are formed with a width α,a height h and a space pt between the adjacent partition walls 113 a.

In the mediums 13, 14, for example, a) an electrostatic latent imagecorresponding to the image to be formed is directly formed on the firstsubstrate 111, or b) an image carrier carrying the electrostatic latentimage corresponding to the image to be formed is contacted with (or madeclose to) the first substrate 111. Thereby an image can be displayed byapplying to the developer DL an electric field for driving thedeveloping particles. The second substrate 112 may be set to a groundpotential, if necessary.

Reversible Image Display Mediums 15, 15′

FIG. 10(A) and FIG. 10(B) show further examples of the reversible imagedisplay medium.

The reversible image display medium 15 (15′) shown in FIG. 10 (A) (FIG.10(B)) is identical with the medium 13 (14) except that an electricallyconductive film 112A is formed on an outer surface of the secondsubstrate 112 in the medium 13 (14).

The medium 15 (15′) is identical in other respects with the medium 13(14). In FIG. 10(A) and FIG. 10(B), the same components and parts as inthe medium 13 (14) are indicated with the same reference symbols.

In image display by the mediums 15, 15′, for example, the electricallyconductive film 112A is set to a ground potential or like properpotential, and a) an electrostatic latent image corresponding to theimage to be formed is directly formed on the first substrate 111, or b)an image carrier carrying the electrostatic latent image correspondingto the image to be formed is contacted with (or made close to) the firstsubstrate 111, whereby an image can be displayed by application ofelectric field for driving the developing particles to the developer DL.

Optionally the second substrate 112 may be made electrically conductiveby dispersing an electrically conductive material, instead of provisionof the electrically conductive film 112A and may be set to a groundpotential or like proper potential.

Each of the image display mediums already described with reference tothe drawings and the image display method utilizing the mediums canrepeat the image display and image erasure.

The developing particles WP and BP are contained in the cell, and it isnot necessary to supply externally the developer into the cell. Thereby,it is possible to suppress significantly the use of medium such as papersheets and consumable materials such as developer, which are requiredfor image display in the prior art. Since a heat energy for melting andfixing the toner onto the medium is not required in contrast to theconventional image display, the image forming energy can be reduced.Accordingly, it is possible to satisfy the present demand for reductionin environmental loads.

Since each of the mediums 11 to 15′ employs the dry developer DLincluding developing particles WP and BP of different colors, one kindof the developing particles WP (or BP) can hide the other kind ofdeveloping particles BP (or WP) to a higher extent so that the imagedisplay in higher contrast can be achieved.

The developing particles WP and BP accommodated in the cell are chargedto the different polarities, respectively, and therefore can be easilymoved for image display by the Coulomb force applied thereto. This alsoimproves the contrast for image display, and can suppress remaining ofthe last image. Further, the image display can be quickly performed, andthe drive voltage for image display can be lowered.

Further, employment of the dry developer DL can suppress settling andcondensation of the developing particles so that lowering in contrastfor the image display can be suppressed, and the image display can bestably performed for a long time. Since the settling and condensation ofthe developing particles are suppressed, remaining of the last displayedimage can be suppressed. Since the change in quality with time issuppressed in the dry developer DL, this also allows stable imagedisplay for a long time.

Any one of the mediums 11 to 15′ can display images in high resolutionas compared with the conventional electrophoretic display.

The mediums except for the medium 11 can display images in higherresolution as compared with the medium 11 in which the resolution isaffected by the size of the pixel electrodes 114 a.

In any one of the mediums 11 to 15′, the strength of electric field (fordriving the developing particles) to be applied to the developer DL maybe adjusted to 0.3 V/μm to 3.0 V/μm at which the developing particlescan smoothly and properly move. Thereby images can be displayed in goodcontrast at a satisfactory image density (density reproducibility).

In the case of forming images using any of the mediums 11 to 15′, thefollowing advantage is given. If the black developing particles BP aremagnetic particles, the oscillating force (magnetic stirring force) canbe affected on the developing particles in the medium by application ofoscillating magnetic field to the developer DL in the medium in imagedisplay, whereby the developing particles are smoothly moved in theelctrostatic field so that the contrast is improved and lower voltagedrive is allowed.

After image display, the application of oscillating magnetic field canbe substantially stopped during application of such electrostatic field,whereby the image display is smoothly performed, the displayed imagesare suppressed from being disturbed in the same oscillating magneticfield and thus the displayed images are more stably held.

The substantial stop of application of oscillating magnetic field can beperformed to assure the retention of displayed images while anelectrostatic field having a strength of at least 0.5 V/μm is stillapplied from outside to the developer (developing particles) after imagedisplay.

After application of electrostatic field, the surface of image displaymedium on the image observation side can be charged to the potentialholding the displayed image, and thereby the displayed image can be morestably held.

The potential holding the displayed image is, e.g. in the range of 10 Vto 100 V in terms of absolute value.

After application of electrostatic field, when the surface of imagedisplay medium is charged to the potential holding the displayed image,it is preferable that the charged polarity of charged potential isadjusted to correspond to the charged polarity of magnetic developingparticles BP.

If black non-magnetic developing particles are used as the blackdeveloping particles BP as described above, the developer can not bemagnetically stirred in image display, but an oscillating force can beapplied to the developer by application of alternating electric field orotherwise. In this case, after application of electrostatic field, thesurface of image display medium on the image observation side can becharged to the potential holding the displayed image, and thereby thedisplayed image can be more stably held. The potential holding thedisplayed image is, e.g. in the range of 10 V to 100 V in terms ofabsolute value.

In image display, a magnetic roller R2 having magnetic poles may beprovided and rotated to apply the oscillating magnetic field, wherebythe developer DL in the cells 116 can be stirred and the developingparticles BP, WP can be easily moved and a low drive voltage is allowed.

When images are displayed using any one of the mediums 11 to 15′, thedeveloper DL of the image display medium can be stirred before imagedisplay on the medium to initialize the medium, whereby images can bedisplayed on the initialized medium.

The image display medium can be initialized as stated above before imagedisplay, whereby the displayed images can be erased and remaining ofimages can be suppressed so that new images can be displayed with higherquality.

The developer can be stirred by such initializing processing, resultingin increased charge quantity of developing particles and instabilization, and thus in high quality image display.

It is preferred to apply alternating electric field to the developer DLof the reversible image display medium in the initializing processing.The developer DL can be efficiently stirred by applying the alternatingelectric field to the developer DL.

In the case of displaying images by forming an electrostatic latentimage on the surface of reversible image display medium, the remainingelectrostatic latent image on the surface of the medium can be erased byapplying alternating electric field prior to formation of image, wherebyremaining of last images can be suppressed and images of higher qualitycan be formed.

When an alternating electric field is applied to the developer forinitialization, the strength of alternating electric field to be appliedto the developer, more specifically to the space in the cells 116accommodating the developer in the image display medium is, forinstance, is 0.5 V/μm or higher.

An upper limit of strength of the electric field to be applied ispreferably about 2.0 V/μm or lower although not limited thereto.

The frequency of the alternating electric field to be applied ispreferably 5 kHz or less.

The alternating electric field may be applied to meet the followingcondition: (the frequency [Hz] of alternating electric field)×(the time[sec] for application of alternating electric field)=20 or more.

When images are displayed using any one of the mediums 11 to 15′, amagnetic field may be affected on the developer DL from outside beforeimage display on the image display medium (before image display step)and/or in image display (in image display step) to apply a stirringforce to the developer. For example, it is possible to use the deviceIN1 for applying a magnetic stirring force for the medium 11 as shown byway of example in FIG. 19.

More specifically, the magnet plate MG1 with alternately arranged S andN magnetic poles laid under the second substrate 112 of the medium 11 isreciprocatingly moved in parallel with the medium 11 by a drive deviceDR1. Thereby an oscillating magnetic field is applied to the developerDL in the medium so that the developer DL containing the magneticdeveloping particles BP is stirred. This can erase the remaining of thelast images if any, can suppress the occurrence of remaining images, canprevent uneven distribution of developing particles in the medium, canachieve uniformity of distribution of the particles and can bring aboutthe desired charged state of developing particles. These factors resultin image display in good quality.

The voltage for image display is applied across the electrodes 114 a and115 opposed to each other, and the magnet plate MG1 is reciprocatinglymoved, whereby the oscillating magnetic field is applied to thedeveloper DL in the medium. The magnetic stirring force is applied tothe developer DL containing the magnetic developing particles BP,thereby facilitating the movement of developing particles and achievingmore rapid image display in high quality by lower voltage drive.

When images are displayed by application of a voltage between theelectrodes 114 a and 115 opposed to each other as in the medium 11, itis unnecessary to consider the occurrence of frictionally chargingbetween the magnet plate MG1 and medium substrate. Accordingly themagnet plate may contact the medium substrate

In respect of any one of the mediums 12 to 15′, the same advantage isafforded by applying a magnetic stirring force to the developer DLbefore and/or in image display using a proper device for applying amagnetic stirring force.

The term “magnet plate” used herein includes various kinds of plateshaving properties such as rigidity or elasticity, and shapes such asthin sheets.

Now, description is given below on examples of image display operationusing mediums 12, 12′, 13, 14, 15 and 15′, as well as on image formingapparatuses.

The image forming apparatus shown in FIG. 11 includes a photosensitivedrum PC which is driven to rotate in the direction of an arrow in thedrawing. Provided around the photosensitive drum PC are a scorotroncharger CH, a laser image exposing device EX, and an eraser lamp IR. Theelectrode roller R1 which is driven to rotate is provided under the drumPC. The electrode roller R1 is a developing electrode roller for formingan electrostatic field for image display. The electrode roller R1 can besupplied with a bias voltage from the power source PW1 and may beinternally provided with a rotary magnetic pole roller R2 which isdriven to rotate in a direction opposite to that of the roller R1 (orwhich is driven to rotate reciprocatingly).

After the surface of the drum PC is charged by the charger CH, imageexposure is performed on the charged region by the laser image exposingdevice EX to form an electrostatic latent image E1 on the drum PC. Onthe other hand, the electrode roller R1 is supplied with a bias voltagefrom the power source PW1. Optionally the electrode roller R1 may be setto a ground potential.

Then, e.g. the medium 13 or 14 is sent between the drum PC and theelectrode roller R1 in synchronization with the electrostatic latentimage E1 on the drum PC. In this operation, the surface of the medium 13(14) is uniformly charged by the charger CRH such as a corona charger tocarry a predetermined potential.

In this way, an electrostatic field is formed based on the electrostaticlatent image E1 and applied to the developing particles BP, WP of thedeveloper DL accommodated in the cells 116 of the medium 13 (14),whereby the developing particles are moved by the Coulomb force exertedbetween the electrostatic field and the charged developing particles.Subsequently the white and black particles WP, BP mingled as shown inFIG. 7(A) or FIG. 8(A) are moved according to the electric field asshown in FIG. 7(B) or FIG. 8(B), whereby images can be displayed in thepredetermined contrast.

After image display as described above, the charges on the surface ofthe photosensitive drum PC are erased by the eraser lamp IR to makeready for the next printing.

It is not essential that the surface of the medium 13 (14) be charged bythe charger CRH.

When the developer DL contains magnetic developing particles in imagedisplay, e.g. black developing particles BP are magnetic, the magneticpole roller R2 is provided and rotated whereby the developer DL in thecells 116 is stirred and the developing particles BP, WP become easilymovable to accomplish display of better images with lower drive voltage.

If the developer DL contains magnetic developing particles in the medium11 shown in FIGS. 1 and 2, the rotary magnetic pole roller R2 is usableas indicated with a chain line in FIG. 2.

The magnet plate MG1 with alternately arranged S and N magnetic polesmay be laid under the second substrate 112 of the medium 11 instead ofthe rotary magnetic pole roller R2 and may be oscillated in parallelwith the medium 11 by the drive device DR1 to form an oscillatingmagnetic field.

Optionally, a magnet plate MG with alternately arranged S and N magneticpoles may be provided downstream in a medium feed path as shown with achain line in FIG. 11 instead of the rotary magnetic pole roller R2.

By application of oscillating magnetic field after application ofelectrostatic field based on the electrostatic latent image E1, theformed images would be likely to become disturbed. In such case, it isdesirable to substantially stop the oscillating magnetic field duringthe application of the electrostatic field.

This matter will be described with reference to FIG. 19. In imagedisplay, the oscillating magnetic field is formed by oscillation of themagnet plate MG1, and after image display, the oscillation of the magnetplate MG1 is stopped after which the application of voltage across theopposite electrodes 114 a and 115 can be stopped.

Images can be formed, for example, using the image forming apparatusshown in FIG. 16. The image forming apparatus shown in FIG. 16 is amodification of the image forming apparatus shown in FIG. 11. In theimage forming apparatus shown in FIG. 16, the rotary magnetic poleroller R2 and a medium-supporting roller RX provided downstream thereofare arranged as opposed to the photosensitive drum PC in the medium feedpath. For example, the medium 13 (14) is passed between the drum PC andthe roller R2 and between the drum PC and the roller RX insynchronization with the electrostatic latent images E1 formed on thedrum PC. During the passage, the medium 13 (14) is contacted with theelectrostatic latent image E1 formed on the drum PC between the rollerR2 and the roller RX, and the substrate 112 of the medium 13 (14) issupplied with a proper bias voltage for forming images from the powersource PW1′ via the electrode Eb.

According to this image displaying method (image forming method), themedium 13 (14) displays images as it is passed between the drum PC andthe roller R2 which provides an oscillating magnetic field. In thiscase, the electrostatic field is continuously applied based on theelectrostatic latent image E1 until the medium 13 (14) is separated fromthe drum PC at a portion of the roller RX. The application ofoscillating magnetic field is substantially stopped (eliminated) at aportion of the roller RX, while the application of electrostatic fieldis still applied. In this way, the formed images are prevented fromdisturbance in the oscillating magnetic field of the roller R2.

The substantial stop of application of oscillating magnetic field isdone, for example, while the electrostatic field contributing to theimage display is applied to the developer at a strength of at least 0.5V/μm although not limited thereto.

In FIG. 16, the same components and parts as in the image formingapparatus of FIG. 11 are indicated with the same reference symbols as inFIG. 11.

When the application of electrostatic field is stopped after imagedisplay in forming images with any of mediums 11 to 15′ (e.g. whenapplication of voltage across the opposite electrodes 114 a and 115 isstopped after image display in forming images with the medium 11 shownin FIGS. 1 and 2), the surface of the reversible image display medium onthe image observation side (surface of the substrate on the imageobservation side) may be charged to a potential holding the displayedimages to stably retain the displayed images. In this operation, themedium may be charged to carry a potential of 10 V to 100 V such thatthe charged polarity corresponds to the charged polarity of the magneticdeveloping particles BP.

FIG. 17 shows an example of such charger. The charger of FIG. 17 has thestructure that, for example, while the medium 11 is transported assupported by a supporting roller SR, the surface of the medium 11 on theimage observation side is charged to carry a potential holding thedisplayed images by being rubbed with a charging brush BR which containswater and which is supplied with a predetermined voltage from the powersource PWX.

Further, as shown in FIG. 18(A), for example, an initializing device INincluding a device for application of alternating electric field may beprovided upstream of the image forming region on the feed path of medium13 (14). The initializing device IN includes a brush charger ch1connected to an AC power source PWa. The electrode Eb is provided asopposed to the brush charger ch1 such that the second substrate of themedium 13(14) (underside substrate in the drawing) is set to a biaspotential. To the electrode Eb is connected the bias power source PWb.Optionally the electrode Eb may be grounded.

The central value of AC voltage supplied from the brush charger ch1 isidentical with the bias voltage to be applied to the electrode Eb.

The developer in the medium 13 (14) is stirred by applying analternating electric field to the medium 13 (14) prior to image displayusing the initializing device IN, whereby the images previously formedcan be erased and the electrostatic latent image can be removed from themedium to achieve initialization.

The brush charger ch1 may be replaced by a corona charger ch2 shown inFIG. 18(B), a brush roller charger ch3 shown in FIG. 18(C) or a rollercharger ch4 shown in FIG. 18(D). In the illustrated modifications, aroller electrode rb is used as an electrode as opposed to the brushroller charger ch3 or the roller charger ch 4.

Using the mediums 12, 12′ or the mediums 15, 15′, image display can bealso achieved by the image forming apparatus shown in FIG. 11 or FIG.18. When images are displayed on the medium 12, 12′, 15 or 15′, thefollowing electrodes or the like may be grounded or supplied with a biasvoltage: the second electrode 115 in the medium 12, the pixel electrodes114 a in the medium 12′ and the electrically conductive film 112A in themediums 15, 15′.

The image forming apparatus shown in FIG. 20(A) includes thephotosensitive drum PC which is driven to rotate in the direction of anarrow in the drawing. Provided around the photosensitive drum PC are ascorotron charger CH, a laser image exposing device EX, and an eraserlamp IR. The electrode roller R1 which is driven to rotate is providedunder the drum PC. The electrode roller R1 is a developing electroderoller for forming an electrostatic field for image display. Theelectrode roller R1 is supplied with a bias voltage from the powersource PW1 and may be internally provided with a rotary magnetic poleroller R2 which is driven to rotate in a direction opposite to that ofthe roller R1 (or which is driven to rotate reciprocatingly) . Theroller R2 is a kind of magnetic field-generating member such that inimage display, a magnetic stirring force is applied to the developer inthe medium by applying the oscillating magnetic field to the developer.The roller R2 forms an example (IN2) of the device for applying amagnetic stirring force for use in image display.

A rotary magnetic pole roller R3 is disposed upstream of the roller R1in the direction of transporting the image display medium. The roller 3is also a magnetic field-generating member which before image display,applies a magnetic stirring force to the developer in the medium byapplying the oscillating magnetic field thereto to thereby initializethe medium. The roller R3 forms the device IN3 for applying a magneticstirring force which is operated before image display. Optionally acharger CRH (a corona charger although not limited thereto) may beprovided, upstream of the roller R1 and, e.g., downstream of the rollerR3, to uniformly charge the surface of the medium before image displayto carry a predetermined potential.

In the foregoing image forming apparatus, the surface of thephotosensitive drum PC is charged by the charger CH and the imageexposure is performed to the charged region of the drum PC by the imageexposing device EX to form an electrostatic latent image E1 on the drumPC. On the other hand, the electrode roller R1 is supplied with a biasvoltage from the power source PW1. Optionally the electrode roller R1may be grounded to carry the ground potential.

Then, e.g. the medium 13 or 14 is sent between the drum PC and theelectrode roller R1 in synchronization with the electrostatic latentimage E1 on the drum PC. An oscillating magnetic field is applied to thedeveloper DL in the medium 13 (14) by the rotary magnetic pole rollerR3, whereby the developer is stirred for erasure of the previouslydisplayed images, i.e. for initialization. Optionally the surface of themedium 13 (14) is uniformly charged by the charger CRH to carry thepredetermined potential.

As the medium 13 (14) arriving in this way is passed between the drum PCand the electrode roller R1, an electrostatic field is formed based onthe electrostatic latent image E1, which acts on the developingparticles BP, WP of the developer DL accommodated in the cells 116.Thereby the developing particles are moved by a Coulomb force exertedbetween the electrostatic field and the charged developing particles. Inthis operation, the developing particles in the medium is made easilymovable due to the oscillating magnetic field applied by the rotarymagnetic pole roller R2.

Thus, the white and black particles WP, BP mingled in the developer DLas shown in FIG. 7(A) or FIG. 8(A) are easily moved to bring them to thestate of white and black particles WP, BP according to the electricfield as shown in FIG. 7(B) or FIG. 8(B). Thereby images can bedisplayed in the predetermined contrast.

After image display as described above, the charges on the surface ofthe photosensitive drum PC are erased by the eraser lamp IR to makeready for the next printing.

Instead of the magnetic pole roller R2, for example, a magnet plate MG2may be disposed downstream of the photosensitive drum PC as shown inFIG. 20(B).

The magnet plate MG2 is a kind of magnetic field-generating member whichforms another example (IN4) of the device for applying a magneticstirring force. As shown in FIG. 26, the magnet plate MG2 (designated MGin FIG. 26) is one in which S and N magnetic poles are alternatelyarranged in the direction of transporting the medium (designated D inFIG. 26, and the cell is designated CEL) . The medium 13 (14) is movedover the magnet plate MG2 so that an oscillating magnetic field isapplied to the developer in the medium, thereby making the developingparticles easily movable. The magnet plate. MG2 may be stationarilyarranged or may be reciprocatingly oscillated in the direction oftransporting the medium.

Instead of the magnet plate MG2, the magnetic pole roller R2 may beprovided in or near the position of the plate MG2. The magnetic poleroller R2 for image display and the magnetic pole roller R3 for usebefore image display are of the type as shown in FIG. 30(A) and FIG.30(B) (designated RG in FIG. 30(A) and FIG. 30(B)). These rollers have Nand S magnetic poles alternately arranged in the direction of rotationof the roller.

The magnet plate MG1 shown in FIG. 19, the magnet plate MG2 shown inFIG. 20(B), and the magnetic pole rollers R2, R3 are magneticfield-generating members having magnetic poles arranged in onepredetermined direction when the surface of the magneticfield-generating members and the medium are relatively moved in thepredetermined direction.

When the magnet plate (such as MG1 and MG2) is used, the magneticdeveloping particles BP tend to be drawn toward one of the twosubstrates in the medium, and are oscillated according to the relativemovement of the magnet plate and the medium in the state as if theparticles BP would form chains of particles as shown in FIG. 23(A) toFIG. 23(D) and FIG. 24(A), whereby the developer is stirred.

In the case of the image forming apparatus shown in FIG. 20(B), forexample, the medium 13 (14) is allowed to display images as follows.

After initializing the medium 13 (14), the electrostatic latent image istransferred to the surface of the medium 13 (14) after which the medium13 (14) carrying the electrostatic latent image is passed over themagnet plate MG2, whereby ultimately the desired images can bedisplayed.

While the developing particles BP, WP of the medium 13 (14) are movedwith difficulty in the stage of the formation of electrostatic latentimage, the developer in the cells 116 receives a magnetic stirring forcesuccessively as the medium 13 (14) is passed over the magnet plate MG2or in other words, as the relative movement is made between the medium13 (14) carrying the electrostatic latent image and the magnet plate MG2as schematically shown in FIG. 25(A) to FIG. 25(C). Thereby images areformed sequentially in the cells.

Other modifications of the image forming apparatus shown in FIG. 20(A)will be described with reference to FIG. 21(A) to FIG. 21(C) and FIG.22(A) to FIG. 22(C).

The image forming apparatus shown in FIG. 21(A) differs from that ofFIG. 20(B) in that the magnet plate MG3 is used instead of the rotarymagnetic pole roller R3 for use before image display. The magnet plateMG3 forms a device IN5 for applying a magnetic stirring force. The imageforming apparatus of FIG. 21(A) is virtually identical in other respectswith the apparatus of FIG. 20(A) and FIG. 20(B). In FIG. 21(A), the samecomponents and parts as in the apparatus of FIG. 20(A) are indicatedwith the same reference symbols as in FIG. 20(A). The magnet plate MG3may be stationarily arranged or may be reciprocatingly oscillated in atravelling direction of the medium 13 (14).

The image forming apparatus shown in FIG. 21(B) differs from that ofFIG. 21(A) in that the magnet plate MG2 for use in image display step isreplaced by a magnetic field-generating member MG4 in the shape of abelt which is driven to rotate.

The image forming apparatus shown in FIG. 21(C) differs from that ofFIG. 21(A) in that the electrode roller R1 is internally provided with arotary magnetic pole roller R2.

The image forming apparatus shown in FIG. 22(A) differs from that ofFIG. 21(A) in that the magnet plates MG3 and MG3′ are opposed to bothsides of the medium 13 (14) before image display and the magnet platesMG2 and MG2′ are opposed thereto in image display step to increase theeffect of stirring the developer.

The image forming apparatus shown in FIG. 22(B) differs from that ofFIG. 21(B) in that in image display step, the belt-like magneticfield-generating member MG4 and the magnet plate MG2′ are opposed to themedium 13 (14).

The image forming apparatus shown in FIG. 22(C) differs from that ofFIG. 21(C) in that the magnet plates MG3 and MG3′ are opposed to bothsides of the medium 13 (14) before image display.

The magnet plates MG2′, MG3′ may be stationarily arranged or may bereciprocatingly oscillated in a travelling direction of the medium 13(14).

When magnetic field-generating members such as a magnet plate areopposed to both sides of the medium, the magnetic developing particlesBP tend to be drawn in the medium toward each of the two substrates, andare oscillated according to the relative movement of the magneticfield-generating members and the medium in the state as if the particleswould form chains of particles on each substrate as shown in FIG. 24(B),whereby the developer is more effectively stirred than when the magneticfield-generating member is opposed only to one side of the medium.

It is possible to increase the stirring effect of developer by makingvarious arrangements of magnetic poles in the magnetic field-generatingmembers such as the magnet plate as opposed to both sides of the medium13 (14).

Using the mediums 12, 12′ and 15, 15′, images can be formed by the imageforming apparatuses shown in FIG. 20, FIG. 21(A) to FIG. 21(C), and FIG.22(A) to FIG. 22(C). When images are formed on these mediums, thefollowing electrodes or film may be grounded or supplied with a biasvoltage: the second electrode 115 in the medium 12, the pixel electrode114 a in the medium 12′ and the electrically conductive film 112A in themediums 15, 15′.

When used on mediums for image display based on an electrostatic latentimage, the following magnet plates and member should be consideredregarding the magnet plates MG2, MG2′, MG3, MG3′, and the belt-likemagnetic field-generating member MG4, especially the magnet plates MG2′,MG3′ as opposed to the substrate on the side of forming an electrostaticlatent image. These magnet plates or member are disposed in a positionremote from the substrate to avert the undesired frictional chargingwith the substrate.

Description is given to other examples of the arrangement of magneticpoles in the magnet plates and the rotary magnetic pole roller withreference to FIGS. 27 to 29 and FIG. 31(A) and FIG. 31(B).

FIG. 27 to FIG. 29 show examples of the arrangement of magnetic poles inthe magnet plates.

When the surface of the magnet plate MG and the reversible image displaymedium D are relatively moved in one predetermined direction and in adirection across the predetermined direction, the magnetic poles of themagnet plate MG may be arranged in the direction across thepredetermined direction as shown in FIG. 27. In this case, the magnetplate MG may be reciprocatingly oscillated in a direction Sw across therelatively travelling direction Fw of the reversible image displaymedium D (e.g. direction vertical thereto).

When the surface of magnet plate MG and the reversible image displaymedium D are relatively moved in one predetermined direction, the magnetplate MG may have at least two rows of magnetic poles such arranged in adirection across the predetermined direction that in two adjacent rowsof magnetic poles, the positions of N and S magnetic poles are displacedfrom each other in the direction of arrangement of magnetic poles asshown in FIG. 28. The magnet plate MG may be stationarily arranged ormay be reciprocatingly oscillated in the direction Sw (e.g. a directionvertical thereto) across the relatively travelling direction Fw of themedium D.

When the surface of magnet plate MG and the reversible image displaymedium D are relatively moved in one predetermined direction, themagnetic poles of the magnet plate MG may be disposed in a direction atan angle to the predetermined direction as shown in FIG. 29. The magnetplate MG may be stationally arranged or may be reciprocatinglyoscillated in the relatively travelling direction Fw of the medium Dand/or in the direction Sw (e.g. a direction vertical thereto) acrossthe travelling direction Fw.

FIG. 31(A) and FIG. 31(B) show an example of arrangement of magneticpoles in the rotary magnetic pole roller. The roller RG is such thatwhen the roller and the medium D are relatively moved in onepredetermined direction, the magnetic poles may be disposed in adirection at an angle to the predetermined direction. Strips of Nmagnetic poles and strips of S magnetic poles are alternately helicallyarranged.

The foregoing magnet plates and the magnetic pole roller can be alsoused for any of the device for applying a magnetic stirring force foruse before image display and the device for applying a magnetic stirringforce for use in image display.

Magnetic poles can be variously arranged in the magneticfield-generating member MG4 in the shape of a belt shown in FIG. 21(B).

The image forming apparatus shown in FIG. 12(A) includes a directelectrostatic latent image forming device CR2 of an ion flow type. Thedevice CR2 includes a corona ion generating portion c2 for generatingcorona ions, a write electrode e2 for leading the corona ions generatedby the ion generating portion onto the surface of, e.g., the firstsubstrate 111 in the medium 13 (or 14), a write electrode controlcircuit f2 for applying to the write electrode e2 the voltage, which isused for leading the positive or negative corona ions to the pixelcorresponding portion on the surface of the substrate 111 in accordancewith the image to be displayed.

The corona ion generating portion c2 includes a shield casing c21 and acorona wire c22, which is stretched in the casing c21. The corona wirec22 is formed of, e.g., gold-plated tungsten wire of 60 μm to 120 μm indiameter. A power source Pc2 applies a positive or negative voltage (4kV to 10 kV) to the wire c22 for generating the corona ions.

The write electrode e2 is opposed to a portion of the shield casing c21,which faces the first substrate 111 of the medium 13 (or medium 14). Thewrite electrode e2 is formed of upper and lower electrodes e21 and e22,and is provided at its center with a hole, through which the corona ionscan flow.

The electrode control circuit f2 includes a control power source Pc21, abias power source Pc22 and a control portion f21. The control portionf21 can apply to the electrodes e21 and e22 the ion leading voltagescorresponding to the polarity of the ions to be led toward the medium13.

Under the control by the control portion f21, the positive and negativevoltages are applied to the upper and lower electrodes e21 and e22,respectively, whereby the positive corona ions can be led to the medium(FIG. 12(A)). By applying the negative and positive voltages to theupper and lower electrodes e21 and e22, respectively, the positivecorona ions can be confined (FIG. 12(B)).

The electrode roller R1 is opposed to the write electrode e2, and issupplied with a positive bias voltage from the power source PW1 or theroller R1 is grounded. The roller R1 is internally provided with amagnetic pole roller R2, which is driven to rotate.

The surface of the medium 13 (or 14) is uniformly charged to apredetermined potential by a charger such as corona charger and thecharged medium 13 (14) is moved relatively to the device CR2. At thesame time, the electrode roller R1 is driven to rotate in the mediumfeed direction, and the magnetic pole roller R2 is rotated in theopposite direction. In accordance with the instruction by the controlportion f21, positive corona ions are led to the predetermined pixelcorresponding portion corresponding to the image to be displayed amongthe plurality of pixel corresponding portions on the surface of thefirst substrate 111, as shown in FIG. 12(A), and outflow of the ions areprevented for the other pixels as shown in FIG. 12(B). Thus the imagedisplay on the medium 13 (or 14) can be performed as shown in FIGS. 7(B)and 8(B).

Before image display, the medium can be initialized with theinitializing devices shown, by way of example, in FIG. 18(A) to FIG.18(D).

Before image display, any one of the above-exemplified devices forapplying a magnetic stirring force can be used which would pose noproblem.

It is not essential that the surface of the medium 13 (or 14) ispreviously charged. The discharging wire c22 in the device CR2 may bereplaced with solid discharging elements.

Using the mediums 12, 12′ or the mediums 15, 15′, image display can beachieved by such image forming apparatus. When images are displayed onthese medium 12, 12′, 15 or 15′, the following electrodes or the likemay be used instead of the electrode roller R1 and may be grounded orsupplied with a bias voltage: the second electrode 115 in the medium 12,the pixel electrodes 114 a in the medium 12′ and the electricallyconductive film 112A in the mediums 15, 15′.

The electrostatic latent image forming device CR2 shown in FIG. 12(A)utilizes the discharging phenomenon. Instead of it, electrostatic latentimage forming devices of various discharging types other than the abovemay be utilized.

The image forming apparatus shown in FIG. 13 includes a directelectrostatic latent image forming device CR3 of the multi-stylus type.The device CR3 includes a multi-stylus head H3 having a plurality ofelectrodes e3, which are arranged in the main scanning direction of,e.g., medium 15 (or 15′), and are arranged close to the first substrate111. A signal voltage is applied to each electrode e3 for applyingelectrostatic latent image charges to the pixel corresponding portion onthe surface of the first substrate 111 in accordance with the image tobe displayed. The medium 15 (or 15′) is transported relatively to thehead H3, e.g., while applying a bias to the conductive film 112A of thesecond substrate 112 or the film 112A is grounded so that the imagedisplay is performed.

Before image display, the medium can be initialized with theinitializing devices shown, by way of example, in FIG. 18(A) to FIG.18(D).

Before image display, any one of above examples of the devices forapplying a magnetic stirring force can be used which would pose noproblem.

Using the mediums 12, 12′, images can be formed by the foregoing imageforming apparatus. In this case, the second electrode 115 of the medium12 and the electrodes 114 a of the medium 12′ may be supplied with abias voltage, when so required.

In the mediums 13, 14, the outer surface of the second substrate 112 maybe supplied with a bias voltage or it may be contacted with an externalelectrode which can be grounded, whereby images can be displayed by theforegoing image forming apparatus.

The image forming apparatus shown in FIG. 14 includes a directelectrostatic latent image forming device CR4 of the charge injectiontype. The device CR4 is of a multi-stylus type, and has an electrostaticrecord head H4, in which a plurality of record electrodes e4 arearranged in the main scanning direction of the medium, and neighboringcontrol electrodes e41 are arranged close to the record electrodes e4.This head is located, e.g., near the medium, and the control electrodese41 of the head H4 are successively and sequentially supplied with avoltage nearly equal to half the voltage (record voltage) required forthe image recording. Also, the record electrodes e4 are supplied withthe image signal voltage nearly equal to half the record voltage.Thereby, the electrostatic latent image can be formed on the mediumlocated immediately under the record electrode.

Described below are specific examples of the developing particles andthe developer, specific examples of the reversible image display mediumand experimental examples of image display using them.

(I) Examples Illustrating the Adjustment of Strength of Electric Fieldfor Driving the Developing Particles

<Developing Particles and Developer>

White Developing Particles WP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofthermoplastic polyester resin (softening point: 121° C., Tg: 67° C.), 40parts by weight of titanium oxide (Ishihara Sangyo Kaisha, Ltd.: CR-50)and 5 parts by weight of salicylic acid-zinc complex(minus-charge-controlling agent, Orient Chemical Co., Ltd.: BontronE-84). The mixture was further mixed by a twin-screw extruder and thencooled. The mixture was roughly pulverized, then pulverized by a jetmill and classified with wind to obtain white fine powders which havevolume average particle sizes of 12 μm, 15 μm and 19 μm.

To the white fine powders having the above sizes was added 0.3 parts byweight of hydrophobic silica particles (Nihon Aerosil Co., Ltd.: AerosilR-972). Each of the mixture was mixed by a Henschel mixer to preparewhite developing particles WP with a particle size shown below in atable.

Black Developing Particles BP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofstyrene-n-butyl methacrylate resin (softening point: 132° C., Tg: 65°C.), 2 parts by weight of carbon black (Lion Oil & Fat Co., Ltd.:Ketchen Black), 1.5 parts by weight of silica (Nihon Aerosil Co., Ltd.:# 200) and 500 parts by weight of magnetic powder containing magnetite(RB-BL, Titan Kogyo Co., Ltd.). The mixture was further mixed by akneader.

After cooling, the mixture was roughly pulverized by a feather mill,then finely pulverized by a jet mill and classified with wind to obtainblack particles BP which have a volume average particle size of 25 μm.The black developing particles BP are magnetic particles.

Developer DL

The white particles WP (30 g) and the black particles BP (70 g) wereplaced into a polyethylene bottle. The bottle was rotated by a ball millpedestal to perform kneading and mixing for 30 minutes to obtainfollowing developers DL1, DL2 and DL3. White Particles Black ParticlesCharge Developer (particle size) (particle size) quantity (μC/g) DL1 12μm 25 μm −4.5 DL2 15 μm 25 μm −3.6 DL3 19 μm 25 μm −2.5Note:Particle size = volume average particle size

The white developing particles WP were negatively charged and blackdeveloping particles BP were positively charged in the developer.

<Reversible Image Display Medium D1 and Image Display>

A reversible image display medium of the same type as the medium 11having electrodes as shown in FIG. 1 was prepared as follows.

A photoresist was applied to a transparent ITO film on a transparent PET(polyethylene terephthalate) film of 50 μm in thickness as the firstsubstrate 111. Over the photoresist was laid a photomask opened in aspecified pattern which was then irradiated with light, followed bydevelopment and etching. Then the remaining photoresist was peeled offfor removal. A first electrode pattern was formed in which square pixelelectrodes 114 a (see FIG. 4) measuring 1 mm and 1 mm were arranged asin a checkerboard with the squares spaced from each other by 0.1 mm insuch a way that lead portions 110 (see FIG. 4) are disposed between andconnected to the pixel electrodes.

A resist was repeatedly applied to increased thickness to other portionsthan the square electrodes 114 a on the substrate 111 to form agrid-like partition member 113 (see FIG. 3). The partition walls 113 aforming the partition member 113 (see FIGS. 3 and 4) had a thickness(width) α (0.1 mm), a height h (100 μm), and a wall space (1 mm) betweenthe neighboring walls (corresponding to one side of the independentelectrode 114 a).

An ITO film was formed as the second electrode 115 by a sputteringmethod to a thickness of 500 Å over the entire surface of the secondsubstrate 112 formed of a transparent PET film of 50 μm in thickness.

Then, the developer DL was placed into each square cavity surroundedwith the partition wall 113 a of the first substrate 111. The volumeproportion of the developer placed into the cavity was 30% based on thevolume of the cavity.

A photo-curing adhesive 119 a (see FIG. 1) was applied only to the topof the partition member 113 to a small thickness after which the ITOelectrode 115 side of the second substrate 112 was closely laid on thetop. Thereby the adhesive was cured by UV irradiation.

Thereafter, the peripheries of the first and second substrates 111, 112were sealed by an epoxy resin adhesive 119 b (see FIG. 1).

In this way, a medium D1 of the type shown in FIG. 1 was produced.

Image display was performed on the medium D1 using the image formingapparatus shown in FIGS. 1 and 2, and the second electrode 115 was setto carry a ground potential. A negative voltage was applied to theindependent electrodes 114 a which correspond to the pixels to bedisplayed in black, while a positive voltage was applied to theindependent electrodes 114 a which correspond to the pixels to bedisplayed in white. In this manner, each independent electrode 114 a wassupplied with a voltage corresponding to the display data to displayimages.

<Reversible Image Display Medium D2 and Image Display>

A reversible image display medium of the same type as the medium 12′having electrodes as shown in FIG. 6(B) was prepared as follows.

A photoresist was applied to a transparent ITO film on a transparent PET(polyethylene terephthalate) film of 50 μm in thickness as the firstsubstrate 111 as done in the case of the medium D1. The photoresist wasexposed to light, developed and etched, and the remaining photoresistwas peeled off for removal. A first electrode pattern was formed inwhich square pixel electrodes 114 a (see FIG. 4) measuring 1 mm and 1 mmwere arranged as in a checkerboard with the squares spaced from eachother by 0.1 mm in such a way that lead portions 110 (see FIG. 4) aredisposed between and connected to the pixel electrodes.

A resist was repeatedly applied to increased thickness to other portionsthan the square electrodes on the substrate 111 to form a grid-likepartition member 113 having a height of 100 μm.

Then, the developer DL was placed into each square cavity surroundedwith the partition wall on the first substrate 111. The volumeproportion of the developer placed into the cavity was 30% based on thevolume of the cavity.

A photo-curing adhesive 119 a was applied to a small thickness only tothe top of the partition member 113 after which the substrate 112 formedof a transparent PET film of 25 μm thickness was closely laid on thetop. Then the adhesive was cured by UV irradiation.

Thereafter, the peripheries of the two substrates 111, 112 were sealedby an epoxy resin adhesive.

In this way, a medium D2 of the type shown in FIG. 6(B) was produced.

Image display was performed on the medium D2 using an image formingapparatus having a multi-stylus type direct electrostatic latent imageforming device CR3 shown in FIG. 13 and independent electrodes 114 awere set to a ground potential. A voltage corresponding to the displaydata was applied from the side of substrate 112 to display images.

<Reversible Image Display Medium D3 and Image Display>

A reversible image display medium of the same type as the medium 14shown in FIG. 8(A) was prepared as follows.

Transparent PET (polyethylene terephthalate) was molded by thermalpressing molding method to give a substrate 111 having the partitionmember 113 on a base portion. The substrate 111 comprised the baseportion having an average thickness (in other words, an averagethickness of the base portion) of 25 μm and continuous partition walls113 a of a wall thickness a (20 μm) and a height h (100 μm) over thebase portion. A plurality of continuous partition walls were formed asarranged in parallel with each other with a space pt (500 μm) away fromeach other (see FIG. 9).

The developer DL was placed into the continuous grooves(developer-accommodating cell) 116 between the neighboring continuouspartition walls 113 a. The volume proportion of the developer placedinto the cavity was 30% based on the volume of the cavity.

A photo-curing adhesive was applied to a small thickness only to the topof the partition walls 113 a (continuous partition walls 113 a) afterwhich a substrate 112 formed of a transparent PET film of 25 m thicknesscontaining carbon black was closely laid on the top, followed by curingthe adhesive by UV irradiation.

Thereafter, the peripheries of the substrates 111, 112 were heat-sealedto form a medium D3 of the type shown in FIG. 8(A) or FIG. 9.

Using the medium D3 of the type shown in FIG. 8(A), images weredisplayed by an image forming apparatus having an ion-flow type directelectrostatic latent image forming device CR2 shown in FIGS. 12(A) and12(B).

The surface of the first substrate 111 was uniformly charged by a coronacharger (not shown) to carry a negative potential. The second substrate112 of the charged medium was set to a ground potential. Then positivecorona ions were led to the predetermined pixel corresponding portionscorresponding to the image to be displayed among the plurality of pixelcorresponding portions on the surface of the first substrate 111 of themedium. The portions were charged to carry a positive polarity potentialand to the same degree (absolute value) of potential as the abovenegatively charged potential.

Thereby the positive corona ion-charged portions and the non-chargedportions were charged to carry the same degree (absolute value) ofpotential and to give different polarities. Thus, image display wasconducted such that the positive corona ion-loaded portions weredisplayed in white by negatively charged white developing particles WPand the positive corona ion-unloaded portions were displayed in black bypositively charged black developing particles BP.

In the image display, the magnetic pole roller R2 was rotated to stirthe developing particles in the medium, whereby images were smoothlydisplayed.

Using the above-mentioned mediums D1 to D3, image display was performedwith varied strengths (V/μm) of electric field for driving thedeveloping particles which was applied to the developer DL accommodatedbetween the substrates. The contrast and image irregularity of displayedimages were evaluated.

The developer DL1 was used as the developer DL.

The contrast was evaluated by measuring the average image density (Bkav.) of the black portions and the average image density (W av.) of thewhite portions by a reflection densitometer (product of KonicaCorporation, Sakura DENSITMETER PDA-65). The evaluation result wasexpressed in average density ratio (Bk av./W av.). An average densityratio of 5.0 or more was rated as good (o) and a lower value was ratedas poor (x)

To evaluate the image irregularity, the image densities of the whiteportions were measured using the reflection densitometer to give adifference of image density between the maximum value and minimum value.An image density difference of 0.2 or less was rated as good (o) and thevalue of more than 0.2 was rated as not good (x).

The strength of electric field applied to the developing particles wasdetermined based on the following potential difference or potentials:

(i) Medium D1

a potential difference between the electrode 114 a and 115 based on thevoltage applied to the independent electrode 114 a in the medium D1;

(ii) Medium D2

a potential applied to the multi-stylus type direct electrostatic latentimage forming device CR3; and

(iii) Medium D3

a surface potential V of the medium D3 determined by a surface potentialmeter (product of TREK, INC., Model 344).

In the medium D3, the strength of the electric field was determined fromthe following equation based on the surface potential V.E=V/[(t ₁/∈₁)+(t ₂/∈₂)+(t ₃/∈₃)]wherein t₁ is the thickness of the first substrate, ∈₁ is the specificdielectric constant thereof, t₂ is the thickness of the cell 116 layer(substantially a space between the substrates), ∈₂ is the specificdielectric constant of cell layer, t₃ is the thickness of the secondsubstrate and ∈₃ is the specific dielectric constant thereof.

The specific dielectric constant ∈₂ of cell layer is the synthetic valuegiven by calculation from the volume ratio of the developer DL and theair layer in the cell, and the partition wall portions forming the cell.

Table 1 shows the evaluation results of the medium D1. Table 2 shows theevaluation results of the medium D2. Table 3 shows the evaluationresults of the medium D3. FIG. 15 shows the relation between thestrength of electric field for driving the developing particles and thecontrast of images. TABLE 1 reversible image display medium D1 voltagestrength image black portion: of black portion white portionirregularity negative electric maximum minimum average maximum minimumaverage contrast W max. − white portion: field density density densitydensity density density Bk av./W av. W min. positive V V/μm (Bk max.){circle over (1)} (Bk min.) {circle over (2)} (Bk av.) {circle over (3)}(W max.) {circle over (4)} (W min.) {circle over (5)} (W av.) {circleover (6)} {circle over (3)}/{circle over (6)} {circle over (4)} −{circle over (5)} 5 0.05 1.15 0.85 1.00 0.65 0.45 0.55 1.8 X 0.20 ◯ 100.1 1.45 1.35 1.40 0.40 0.25 0.33 4.3 X 0.15 ◯ 20 0.2 1.65 1.58 1.620.35 0.25 0.30 5.4 ◯ 0.10 ◯ 30 0.3 1.67 1.50 1.59 0.30 0.24 0.27 5.9 ◯0.06 ◯ 50 0.5 1.65 1.52 1.59 0.28 0.23 0.26 6.2 ◯ 0.05 ◯ 100 1 1.63 1.491.56 0.26 0.22 0.24 6.5 ◯ 0.04 ◯ 200 2 1.61 1.50 1.56 0.27 0.22 0.25 6.3◯ 0.05 ◯ 300 3 1.58 1.45 1.52 0.34 0.25 0.30 5.1 ◯ 0.09 ◯ 350 3.5 1.541.68 1.61 0.54 0.23 0.39 4.2 X 0.31 X 400 4 1.62 1.65 1.64 0.52 0.250.39 4.2 X 0.27 X

TABLE 2 reversible image display medium D2 voltage strength image blackportion: of black portion white portion irregularity negative electricmaximum minimum average maximum minimum average contrast W max. − whiteportion: field density density density density density density Bk av./Wav. W min. positive V V/μm (Bk max.) {circle over (1)} (Bk min.) {circleover (2)} (Bk av.) {circle over (3)} (W max.) {circle over (4)} (W min.){circle over (5)} (W av.) {circle over (6)} {circle over (3)}/{circleover (6)} {circle over (4)} − {circle over (5)} 10 0.09 1.32 0.92 1.120.55 0.36 0.46 2.5 X 0.19 ◯ 20 0.18 1.48 1.25 1.37 0.32 0.25 0.29 4.8 X0.07 ◯ 30 0.28 1.65 1.52 1.59 0.35 0.22 0.29 5.6 ◯ 0.13 ◯ 50 0.46 1.621.48 1.55 0.25 0.23 0.24 6.5 ◯ 0.02 ◯ 100 0.92 1.61 1.46 1.54 0.23 0.240.24 6.5 ◯ −0.01 ◯ 200 1.85 1.59 1.50 1.55 0.25 0.22 0.24 6.6 ◯ 0.03 ◯300 2.77 1.58 1.43 1.51 0.26 0.23 0.25 6.1 ◯ 0.03 ◯ 350 3.23 1.56 1.381.47 0.35 0.25 0.30 4.9 X 0.10 ◯ 400 3.69 1.52 1.35 1.44 0.48 0.23 0.364.0 X 0.25 X 450 4.15 1.47 1.29 1.38 0.80 0.25 0.53 2.6 X 0.55 X

TABLE 3 reversible image display medium D3 voltage strength image blackportion: of black portion white portion irregularity negative electricmaximum minimum average maximum minimum average contrast W max. − whiteportion: field density density density density density density Bk av./Wav. W min. positive V V/μm (Bk max.) {circle over (1)} (Bk min.) {circleover (2)} (Bk av.) {circle over (3)} (W max.) {circle over (4)} (W min.){circle over (5)} (W av.) {circle over (6)} {circle over (3)}/{circleover (6)} {circle over (4)} − {circle over (5)} 10 0.09 1.30 0.85 1.080.55 0.36 0.46 2.4 X 0.19 ◯ 20 0.17 1.45 1.23 1.34 0.32 0.25 0.29 4.7 X0.07 ◯ 30 0.26 1.60 1.48 1.54 0.35 0.22 0.29 5.4 ◯ 0.13 ◯ 50 0.43 1.591.49 1.54 0.25 0.24 0.25 6.3 ◯ 0.01 ◯ 100 0.86 1.60 1.43 1.52 0.23 0.220.23 6.7 ◯ 0.01 ◯ 200 1.71 1.57 1.43 1.50 0.25 0.22 0.24 6.4 ◯ 0.03 ◯300 2.57 1.55 1.40 1.48 0.26 0.23 0.25 6.0 ◯ 0.03 ◯ 350 3.00 1.52 1.381.45 0.35 0.24 0.30 4.9 X 0.11 ◯ 400 3.43 1.48 1.31 1.40 0.42 0.22 0.324.4 X 0.20 ◯ 450 3.86 1.37 1.23 1.30 0.80 0.25 0.53 2.5 X 0.55 X

The above evaluation results show that a proper strength of electricfield for driving the developing particles is 0.3 V/μm to 3.0 V/μm.

The image evaluation was conducted in the same manner as above inrespect of the mediums containing other developers DL2, DL3, with thesame results that a proper strength of electric field for driving thedeveloping particles is 0.3 V/μm to 3.0 V/μm.

(II) Examples of Application of Oscillating Magnetic Field and theTiming of its Stop and Examples of Charging the Surface of the Medium toCarry a Potential Holding the Images

<Developing Particles and Developer>

White Developing Particles WP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofthermoplastic polyester resin (softening point: 121° C., Tg: 67° C.), 40parts by weight of titanium oxide (Ishihara Sangyo Kaisha, Ltd.: CR-50)and 5 parts by weight of salicylic acid-zinc complex(minus-charge-controlling agent, Orient Chemical Co., Ltd.: BontronE-84). The mixture was further mixed by a twin-screw extruder and thencooled. The mixture was roughly pulverized, then pulverized by a jetmill and classified with wind to obtain white fine powder which has avolume average particle size of 10.1 μm. To the white fine powder wasadded 0.3 parts by weight of hydrophobic silica particles (Nihon AerosilCo., Ltd.: Aerosil R-972). The mixture was mixed by a Henschel mixer toprepare white developing particles WP.

Black Developing Particles BP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofstyrene-n-butyl methacrylate resin (softening point: 132° C., Tg: 65°C.), 2 parts by weight of carbon black (Lion Oil & Fat Co., Ltd.:Ketchen Black), 1.5 parts by weight of silica (Nihon Aerosil Co., Ltd.:# 200) and 500 parts by weight of magnetic powder containing magnetite(RB-BL, Titan Kogyo Co., Ltd.). The mixture was further mixed by akneader.

After cooling, the mixture was roughly pulverized by a feather mill,then finely pulverized by a jet mill and classified with wind to obtainblack magnetic particles BP which have a volume average particle size of25 μm.

Preparation of Developer DL

The white particles WP (30 g) and the black particles BP (70 g) wereplaced into a polyethylene bottle. The bottle was rotated by a ball millpedestal to mix them for 30 minutes to obtain a developer DL. The whitedeveloping particles WP were negatively charged and black developingparticles BP were positively charged.

<Reversible Image Display Medium D1x>

The reversible image display medium D1x is similar to the medium 14shown in FIG. 8(A). Namely in the medium 14, it has a transparentelectrode in common with all pixels on the internal surface of thesecond substrate 112 and an independent electrode for each cell 116 onthe internal surface of the first substrate 111 on the image observationside.

A UV curing resin film of 150 μm in thickness was formed on anelectrically conductive film (a common electrode) formed of ITO(transparent electrode) on a transparent glass substrate 112. Over theresin film was laid a photomask opened in a specified pattern which wasthen irradiated with ultraviolet light, followed by development andwashing. Then, over the electrode of the glass substrate 112 were formeda plurality of continuous partition walls 113 a extending in parallel.The partition wall 113 a had a thickness (width) α (50 μm), a height h(150 μm), and a wall space pt (300 μm) between the neighbouringpartition walls.

Thereafter, the foregoing developer DL was placed into each of thecontinuous groove-like cells between the neighboring partition walls onthe substrate 112. The developer was placed into the cells at a fillfactor of 34 vol. % based on the volume of the cell.

A glass substrate was prepared as the first substrate 111. On the glasssubstrate 111 were formed transparent independent electrodes inpositions corresponding to the cells 116. On the glass substrate 111were also formed lead portions connected to the independent electrodesin positions corresponding to the walls 113 a.

A photo-curing adhesive was applied to a small thickness only to the topof each partition wall 113 a on the substrate 112. The substrate 111 waslaid on the adhesive on the partition walls 113 a with the independentelectrodes directed to the cells. Thus the adhesive was cured by UVirradiation. Thereafter, the peripheries of the two glass substrateswere sealed by a sealer.

In this way, a medium D1x which was similar to the medium shown in FIG.8(A) was produced.

<Image Display Using the Medium D1x>

The magnet plate MG1 shown in FIG. 19 was disposed under the secondsubstrate 112 having the common electrode in the medium D1x. Then thesame image forming apparatus as shown in FIG. 1 was connected to theindependent electrodes of the substrate 111 on the image observationside.

The magnet plate MG1 was a rubber magnet plate having a surface magneticforce of 300 gausses. The magnet plate MG1 was capable of achievingvibrational drive in parallel with the medium by a drive device DR1.

In image display, each of the independent electrodes was supplied with+200V or −200V according to the images to be formed. The commonelectrode was set to a ground potential, and the magnet plate MG1 wasoscillated. The oscillating magnetic field was applied to the medium,whereby the stirring force was applied to the developer in the cells.

Thus, after image formation, the oscillation of magnet plate MG1 wasstopped and the oscillating magnetic field was stopped. Thereafter theapplication of voltage to each independent electrode was stopped.

The image density of the images formed in this way was measured fromabove the glass substrate using a reflection densitometer (product ofX-Rite Incorporated, 310 TR, aperture diameter 2 mm). The black displayportion had a reflection density of 1.7 and the white display portionhad a reflection density of 0.2. Therefore, the images were confirmed asgood.

A comparative image display was also performed. In the comparative imagedisplay, after the formation of images, the application of voltage toeach independent electrode was stopped first and then the application ofoscillating magnetic field was stopped.

The image density of the images formed in this way was also measured inthe same manner as above, and it was found that the black displayportion had a reflection density of 0.9 and the white display portionhad a reflection density of 0.7.

This shows that it is preferred to substantially stop the application ofoscillating magnetic field after image display during application ofelectrostatic field for image display.

<Reversible Image Display Medium D2x>

A reversible image display medium of the same type as the medium15′shown in FIG. 10(B) was prepared as follows.

A transparent PET (polyethylene terephthalate) film was pressed bythermal pressing molding method to give a plurality of continuouspartition walls 113 a on a base portion of 25 μm thickness correspondingto the first substrate 111. The continuous partition walls 113 a had awall thickness (width) α (20 μm), a height h (100 μm) and a space pt(200 μm) between adjacent partition walls.

The developer DL was placed into each of the continuous groove-like cell116 between the neighboring partition walls 113 a on the substrate 111.The developer was placed into the continuous groove-like cell at a fillfactor of 34 vol. % based on the volume of the continuous groove-likecell.

A photo-curing adhesive was applied to a small thickness only to the topof the partition walls 113 a of the substrate 111. Then, a PET film of30 μm thickness having an electrically conductive film of aluminum 112Aformed over its external surface by deposition as the second substrate112 was adhered to the adhesive over its internal surface. The adhesivewas cured by UV irradiation to adhere the film.

Thereafter, the peripheries of the first and second substrates 111, 112were heat-sealed.

In this way, a medium D2x of the type shown in FIG. 10(B) was produced.

<(1) Image Display Using the Medium D2x>

Description is given on the image display using the medium D2x(Experimental Examples 1x to 9x) and on comparative image display(Comparative Examples 1x to 11x).

The image forming apparatus shown in FIG. 16 was used for image displayby the medium D2x.

In the image forming apparatus shown in FIG. 16, the surface of thephotosensitive drum PC was charged by the charger CH to carry a negativesurface potential V₀ and the image exposure was performed to the chargedregion by the image exposing device EX to form an electrostatic latentimage E1. On the other hand, V₀/2 was applied from the power source PW1′to the electrically conductive film 112A of the medium D2x fed thereto.

Thus, the medium D2x was sent between the drum PC and the rotarymagnetic pole roller R2 forming the oscillating magnetic field insynchronization with the electrostatic latent image on the drum PC. Anelectrostatic field for image display and an oscillating magnetic fieldwere applied to the medium D2x thus fed so that image display wasperformed successively as the medium D2x was transported. Theimage-displayed portion of the medium D2x was transported as passedbetween the drum PC and the supporting roller RX while it was contactedwith the surface of the drum PC, in other words, the electrostatic fieldbased on the electrostatic latent image E1 was applied but theoscillating magnetic field was decreased.

Herein image display was conducted by changing the magnetic force in aposition A shown in FIG. 16 wherein the medium D2x was separated fromthe drum PC. The magnetic force was varied by changing the surfacemagnetic force of the roller R2 from 300 gausses to 1000 gausses and/orchanging the distance between the roller R2 and the roller RX.

Further, image display was effected by varying the electric fieldstrength to be applied to the developer in the medium D2x due to changeof the charged surface potential of photosensitive drum by the chargerCH. The potential difference V acting on the upper side and underside ofthe medium D2x is a difference between the surface potential ofphotosensitive drum and the bias by the power source PW1′. The electricfield strength E to be applied to the developer in thedeveloper-accommodating cells was employed, which is given bycalculation of the following equation:E=V/[(t ₁/∈₁)+(t ₂/∈₂)+(t ₃/∈₃)]wherein t₁ is the thickness of the substrate on the image observationside, ∈₁ is the specific dielectric constant thereof, t₂ is thethickness of the developer-accommodating cell layer (substantially aspace between the substrates), ∈₂ is the specific dielectric constant ofcell layer, t₃ is the thickness of the opposite substrate and ∈₃ is thespecific dielectric constant thereof.

The specific dielectric constant ∈₂ of cell layer is the synthetic valuegiven by calculation from the volume ratio of the developer DL and theair layer in the cell, and the partition wall portions forming the cell.

Measurements were made of the reflection density (that of black Bk) ofsolid portion (black portion) of formed images and the reflectiondensity (W reflection density) of white portion of the backgroundportion to give a reflection density ratio [B(Bk)/W], whereby the imageswere evaluated. The reflection density was measured using a reflectiondensitometer (product of X-Rite Incorporated, 310 TR, aperture diameter2 mm).

A reflection density ratio (B/W) of 5.0 or more was rated as good (o)and that of less than 5.0 was rated as unsatisfactory in contrast (x).

The results of image evaluation are shown in Table 4 together with themagnetic force (gauss) at a position A and electric field strength E(V/μm) applied to the developer.

The magnetic force in a position A was measured with a gauss metermanufactured by Denshijiki Industry Co., Ltd. TABLE 4 magnetic strengthB/W force in of electric Bk W image evaluation position A fieldreflection reflection density of B/W image (gauss) (V/μm) densitydensity ratio density ratio comparative 200 2 0.95 0.83 1.1 X example 1comparative 200 1 0.95 0.86 1.1 X example 2 comparative 200 0.5 0.980.84 1.2 X example 3 comparative 200 0.3 0.97 0.85 1.1 X example 4comparative 150 2 0.93 0.82 1.1 X example 5 comparative 150 1 0.94 0.81.2 X example 6 comparative 150 0.5 0.95 0.83 1.1 X example 7comparative 150 0.3 0.92 0.81 1.1 X example 8 example 1 100 2 1.35 0.245.6 ◯ example 2 100 1 1.34 0.25 5.4 ◯ example 3 100 0.5 1.32 0.24 5.5 ◯comparative 100 0.3 1.1 0.5 2.2 X example 9 example 4 50 2 1.45 0.2 7.3◯ example 5 50 1 1.42 0.22 6.5 ◯ example 6 50 0.5 1.25 0.23 5.4 ◯comparative 50 0.3 1.15 0.35 3.3 X example 10 example 7 0 2 1.45 0.2 7.3◯ example 8 0 1 1.43 0.21 6.8 ◯ example 9 0 0.5 1.4 0.25 5.6 ◯comparative 0 0.3 1.15 0.35 3.3 X example 11

As clear from Table 4, in Experimental Examples 1x to 9x, the electricfield strength applied to the developer was 0.5 V/μm or more, and themagnetic force was 100 gausses or less in the oscillating magnetic fieldin the position A (gauss) in which the application of electrostaticfield was stopped, and images in good contrast were obtained. On theother hand, in Comparative Experimental Examples 1x to 11x, the electricfield strength applied to the developer was less than 0.5 V/μm or themagnetic force was more than 100 gausses in the oscillating magneticfield in the position A wherein the application of electrostatic fieldwas stopped. Images which were satisfactory in contrast were notobtained

It is evident from the above that good images are obtained when theelectric field strength applied to the developer is preferably 0.5 V/μmor more and preferably the application of oscillating magnetic field issubstantially stopped during application of electrostatic field.

<(2) Image Display Using the Medium D2x>

First, using the image forming apparatus shown in FIG. 16, an imagedisplay was performed on each of a plurality of mediums D2x at a chargedpotential of −400V on the surface of photosensitive drum by the chargerCH and at a bias voltage of −200 from the power source PWl′ with use ofthe roller R2 having a surface magnetic force of 600 gausses. Each ofthe mediums D2x displayed images which showed a Bk reflection density of1.45, W reflection density of 0.2 and reflection density ratio (B/W) of7.3.

The surfaces of the mediums D2x except one medium D2x on the imageobservation side were charged to carry various potentials by the chargershown in FIG. 17. Thereafter the change of images were checked byapplying a mechanical oscillation to the medium D2x.

The mechanical oscillation was applied by applying the mechanicaloscillation 5 times with a weight of 20 g from a height of 45 degrees asa pendulum of 15 cm in radius from the side of the medium D2x.

The change of images was checked after applying the mechanicaloscillation. The Bk reflection density of solid portion of formed imagesand the W reflection density of the background portion were measured,followed by determination of a reflection density ratio [B(Bk)/W]. Thereflection density was measured using a reflection densitometer asdescribed above (product of X-Rite Incorporated, 310 TR, aperturediameter 2 mm).

A reflection density ratio (B/W) of 5.0 or more was rated as suppressedimage change (mark o) and that of less than 5.0 was deemed to sufferdisturbance of images (mark x). The evaluation results are shown inTable 5. TABLE 5 B/W charged Bk W image potential reflection reflectiondensity evaluation [V] density density ratio result — 0.95 0.83 1.1 X 01.45 0.27 5.4 ◯ 50 1.45 0.29 5.0 ◯ 150 0.9 0.25 3.6 X −150 1.45 0.82 1.8X

As apparent from Table 5, the image display medium sent from the imageforming apparatus was increased in image-retaining stability by beingcharged to 100V or less in terms of the absolute value.

Further, it is clear that when the medium was charged to have the samepolarity as the magnetic developing particles, the contrast can beproperly retained.

The same can be said about the mediums of the same type as the mediums11, 12, 12′, 13, 14 and 15.

(III) Examples of Initialization of Medium Before Image Display

<Developing Particles and Developer>

White Developing Particles WP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofthermoplastic polyester resin (softening point: 121° C., Tg: 67° C.), 40parts by weight of titanium oxide (Ishihara Sangyo Kaisha, Ltd.: CR-50)and 5 parts by weight of salicylic acid-zinc complex(minus-charge-controlling agent, Orient Chemical Co., Ltd.: BontronE-84). The mixture was further mixed by a twin-screw extruder and thencooled. The mixture was roughly pulverized, then pulverized by a jetmill and classified with wind to obtain white fine powder which have avolume average particle size of 12 μm. To the white fine powder wasadded 0.3 parts by weight of hydrophobic silica particles (Nihon AerosilCo., Ltd.: Aerosil R-972). The mixture was mixed by a Henschel mixer toprepare white developing particles WP.

Black Developing Particles BP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofstyrene-n-butyl methacrylate resin (softening point: 132° C., Tg: 65°C.), 2 parts by weight of carbon black (Lion Oil & Fat Co., Ltd.:Ketchen Black), 1.5 parts by weight of silica (Nihon Aerosil Co., Ltd.:# 200) and 500 parts by weight of magnetic powder containing magnetite(RB-BL, Titan Kogyo Co., Ltd.). The mixture was further mixed by akneeder.

After cooling, the mixture was roughly pulverized by a feather mill,then finely pulverized by a jet mill and classified with wind to obtainblack magnetic particles BP which have a volume average particle size of25 μm.

Preparation of Developer DL

The white particles WP (30 g) and the black particles BP (70 g) wereplaced into a polyethylene bottle. The bottle was rotated by a ball millpedestal to perform the mixing and stirring for 30 minutes to obtain adeveloper.

The white developing particles WP were negatively charged and blackdeveloping particles BP were positively charged in the developer.

<Reversible Image Display Medium D1y>

A reversible image display medium of the same type as the medium 14shown in FIG. 8(A) was prepared as follows.

A transparent PET (polyethylene terephthalate) film was pressed bythermal pressing molding method to give a plurality of continuouspartition walls 113 a on a base portion of 25 μm thickness correspondingto the first substrate 111. The continuous partition wall 113 a had awall thickness (width) α (30 μm), a height h (100 μm) and a space pt(500 μm) between adjacent partition walls.

The developer DL was placed into each of the the continuous groove-likecells 116 between the neighboring partition walls 113 a on the substrate111. The volume proportion of the developer placed into the continuousgroove-like cell was 30 vol. % based on the volume of the cell.

A photo-curing adhesive was applied to a small thickness only to the topof the partition walls 113 a on the substrate 111. Then a secondsubstrate 112 formed of a transparent PET film of 25 m thicknesscontaining carbon black was closely laid on the adhesive over itsinternal surface. The PET film was adhered thereto by curing theadhesive by UV irradiation.

Thereafter, the peripheries of the substrates 111, 112 were heat-sealed.

In this way, a medium D1y of the type shown in FIG. 8(A) was produced.

<Reversible Image Forming Medium D2y>

A reversible image display medium of the same type as the medium 11 withelectrodes as shown in FIG. 1 was prepared as follows.

A photoresist was applied to a transparent ITO film on the entiresurface of a transparent PET (polyethylene terephthalate) film of 50 μmin thickness as the first substrate. Over the photoresist was laid aphotomask opened in a specified pattern which was then irradiated withlight, followed by development and etching. Then the remainingphotoresist was peeled off for removal. A first electrode pattern wasformed in which square pixel electrodes 114 a (see FIG. 4) measuring 1mm and 1 mm were arranged as in a checkerboard with the squares spacedfrom each other by 0.1 mm in such a way that lead portions 110 (see FIG.4) are disposed between and connected to the pixel electrodes.

A resist was repeatedly applied to increased thickness to other portionsthan the square electrodes 114 a on the substrate 111 to form agrid-like partition member 113 (see FIG. 3). The partition walls 113 aforming the partition member 113 had a thickness (width) α (0.1 mm), aheight h (100 μm), and a wall space (1 mm) (corresponding to one side ofthe independent electrode 114 a).

An ITO film was formed as a second electrode 115 by a sputtering methodto a thickness of 500 Å over the entire surface of the second substrate112 formed of a transparent PET film of 50 μm in thickness.

Then, the developer DL was placed into each square cavity surroundedwith the partition wall of the first substrate 111. The volumeproportion of the developer placed into the cavity was 30% based on thevolume of the cavity.

A photo-curing adhesive 119 a (see FIG. 1) was applied to a smallthickness only to the top of the partition member 113 after which theITO electrode 115 side of the second substrate 112 was laid on the top.Then the adhesive was cured by UV irradiation.

Thereafter, the peripheries of the first and second substrates 111, 112were sealed by an epoxy resin adhesive 119 b (see FIG. 1).

In this way, a medium D2y of the type shown in FIG. 1 was produced.

<Image Display Using the Medium D1y>

A plurality of mediums D1y were provided. Image display was performed asdescribed below using each medium D1y by the image forming apparatusshown in FIG. 18(A).

In formation of each image, the surface of the photosensitive drum PCwas uniformly charged by the charger CH to −900V. The image exposure wasperformed to the charged region by the image exposing device EX and thesurface potential in the exposed region was reduced to about −50V toform an electrostatic latent image E1.

On the other hand, the medium D1y thus fed was supplied with analternating electric field (AC voltage) by the initializing device IN tostir the developer DL in the medium, whereby the medium D1y wasinitialized and the surface of the medium D1y was uniformly charged bythe charger CRH to have a polarity of about +400V opposite to thecharged polarity of the surface of photosensitive drum before imagedisplay.

Subsequently, the medium D1y was passed between the drum PC and theopposite electrode roller (transfer roller) R1 in synchronization withthe electrostatic latent image E1 formed on the drum PC. The roller R1of about 10⁸Ωin resistance was used and a transfer bias voltage of about+1300V was applied to the roller R1 from the power source PW1.

In this way, the electrostatic latent image was transferred to thesurface of the medium D1y at about −300V in the image portion (blackportion) and at about +300V in the background portion (white portion).

The medium D1y carrying the electrostatic latent image as describedabove was passed above the magnet plate MG, whereby the developer DLcontaining magnetic developing particles BP was subjected to agitationby the oscillating magnetic field, thereby facilitating the movement ofdeveloping particles to achieve image display.

Each of the mediums D1y with the images formed as above was initializedunder varied conditions for applying the alternating electric field (ACelectric field) by the initializing device such as the electric fieldstrength, and the frequency and (frequency×application time (period)).

More specifically, the conditions include the electric field strength inthe range of 0.1 V/μm to 1.0 V/μmK, the frequency in the range of 100 Hzto 10 kHz and (frequency×application time) comprising combinations offrequency of 100 Hz to 5 kHz with 5 ms to 200 ms.

The presence or absence of remaining images on the medium D1yimmediately after initialization and uncharged by the charger CRH beforeimage display, i.e. the degree of initialization, was determined.

If the maximum image reflection density and the minimum image reflectiondensity are both in the range of 0.7 to 0.8, it means the sufficienterasure of last images and achievement of the desired initialization(◯), whereas if at least one of the maximum image reflection density andthe minimum image reflection density is outside this range, it meansinsufficiency of initialization (X) . The evaluation results are shownin Tables 6 and 7.

The image density was measured by a reflection densitometer (product ofKonica Corporation, Sakura DENSITMETER PDA-65).

The electric field strength was determined according to the followingequation:E=V/[(t ₁/∈₁)+(t ₂/∈₂)+(t ₃/∈₃)]wherein E is an electric field strength, V is a surface potential of themedium D1y (the measured value obtained by a surface potential meter,product of TREK, INC., Model 344), t₁ is the thickness of the substrateon the observation side, ∈₁ is the specific dielectric constant thereof,t₂ is the thickness of the developer-accommodating cell layer 116(substantially a space between the substrates), ∈₂ is the specificdielectric constant of the cell layer, t₃ is the thickness of thesubstrate on the opposite side and ∈₃ is the specific dielectricconstant thereof.

The specific dielectric constant ∈₂ of the cell layer was 1.5 in termsof synthetic value given by calculating the volume ratio of thedeveloper DL and the air layer in the cell, and the partition wallportions forming the cell. TABLE 6 strength of alternating electricfield frequency dependency frequency(Hz) 4k strength 100 500 1k 2k 3kre- of image image image image image image main- electric densityremain- density remain- density remain- density remain- density remain-density ing field maxi- mini- ing maxi- mini- ing maxi- mini- ing maxi-mini- ing maxi- mini- ing maxi- mini- im- (V/μm) mum mum images mum mumimages mum mum images mum mum images mum mum images mum mum ages 0.11.67 0.20 X 1.65 0.21 X 1.67 0.22 X 1.66 0.22 X 1.63 0.24 X 1.65 0.22 X0.2 1.55 0.21 X 1.51 0.26 X 1.52 0.23 X 1.46 0.26 X 1.55 0.22 X 1.610.24 X 0.3 1.01 0.42 X 0.99 0.40 X 1.03 0.46 X 0.95 0.50 X 1.20 0.55 X1.25 0.58 X 0.4 0.82 0.65 X 0.82 0.63 X 0.81 0.62 X 0.85 0.67 X 0.830.65 X 0.86 0.65 X 0.5 0.79 0.71 ◯ 0.78 0.71 ◯ 0.79 0.71 ◯ 0.78 0.72 ◯0.79 0.73 ◯ 0.78 0.71 ◯ 0.6 0.78 0.70 ◯ 0.77 0.71 ◯ 0.79 0.72 ◯ 0.780.72 ◯ 0.79 0.73 ◯ 0.76 0.72 ◯ 0.7 0.78 0.72 ◯ 0.77 0.72 ◯ 0.76 0.74 ◯0.77 0.73 ◯ 0.77 0.74 ◯ 0.77 0.71 ◯ 0.8 0.77 0.74 ◯ 0.76 0.73 ◯ 0.770.74 ◯ 0.77 0.73 ◯ 0.78 0.73 ◯ 0.77 0.72 ◯ 0.9 0.76 0.75 ◯ 0.77 0.74 ◯0.76 0.73 ◯ 0.76 0.74 ◯ 0.77 0.74 ◯ 0.76 0.72 ◯ 1.0 0.76 0.74 ◯ 0.760.74 ◯ 0.77 0.73 ◯ 0.76 0.74 ◯ 0.78 0.75 ◯ 0.77 0.73 ◯ frequency(Hz) 10kstrength 5k 6k 7k 8k 9k re- of image image image image image image main-electric density remain- density remain- density remain- density remain-density remain- density ing field maxi- mini- ing maxi- mini- ing maxi-mini- ing maxi- mini- ing maxi- mini- ing maxi- mini- im- (V/μm) mum mumimages mum mum images mum mum images mum mum images mum mum images mummum ages 0.1 1.64 0.20 X 1.64 0.20 X 1.66 0.22 X 1.65 0.21 X 1.64 0.22 X1.63 0.22 X 0.2 1.54 0.21 X 1.60 0.21 X 1.59 0.22 X 1.61 0.21 X 1.600.21 X 1.61 0.22 X 0.3 1.02 0.46 X 1.26 0.40 X 1.35 0.35 X 1.40 0.33 X1.55 0.31 X 1.60 0.23 X 0.4 0.88 0.59 X 0.89 0.55 X 0.94 0.44 X 1.100.41 X 1.22 0.36 X 1.55 0.29 X 0.5 0.79 0.70 ◯ 0.81 0.65 X 0.91 0.60 X0.95 0.45 X 1.10 0.43 X 1.35 0.35 X 0.6 0.79 0.70 ◯ 0.81 0.69 X 0.850.65 X 0.91 0.47 X 1.05 0.44 X 1.16 0.41 X 0.7 0.78 0.71 ◯ 0.79 0.69 X0.86 0.63 X 0.91 0.45 X 1.01 0.44 X 1.11 0.41 X 0.8 0.79 0.72 ◯ 0.820.71 X 0.84 0.66 X 0.89 0.47 X 0.99 0.48 X 1.15 0.43 X 0.9 0.78 0.73 ◯0.81 0.68 X 0.81 0.68 X 0.87 0.49 X 0.97 0.47 X 1.13 0.43 X 1.0 0.770.72 ◯ 0.79 0.69 X 0.81 0.66 X 0.88 0.51 X 0.97 0.48 X 1.10 0.42 X

TABLE 7 oscillation dependency strength of electric field: 1 V/μm(fixed)application time(ms) 50 5 10 20 30 40 re- image image image image imageimage main- fre- density remain- density remain- density remain- densityremain- density remain- density ing quency maxi- mini- ing maxi- mini-ing maxi- mini- ing maxi- mini- ing maxi- mini- ing maxi- mini- im- (Hz)mum mum images mum mum images mum mum images mum mum images mum mumimages mum mum ages 100 1.57 0.26 X 1.51 0.27 X 1.41 0.28 X 1.38 0.29 X1.35 0.31 X 1.27 0.33 X 500 1.40 0.29 X 1.26 0.31 X 0.88 0.59 X 0.820.68 X 0.78 0.73 ◯ 0.77 0.74 ◯ 1k 1.28 0.33 X 0.87 0.56 X 0.77 0.73 ◯0.77 0.74 ◯ 0.76 0.74 ◯ 0.77 0.74 ◯ 2k 0.89 0.61 X 0.77 0.73 ◯ 0.76 0.73◯ 0.77 0.73 ◯ 0.78 0.73 ◯ 0.76 0.74 ◯ 5k 0.76 0.74 ◯ 0.77 0.74 ◯ 0.760.74 ◯ 0.78 0.76 ◯ 0.77 0.74 ◯ 0.76 0.74 ◯ application time(ms) 200 6070 80 90 100 re- image image image image image image main- fre- densityremain- density remain- density remain- density remain- density remain-density ing quency maxi- mini- ing maxi- mini- ing maxi- mini- ing maxi-mini- ing maxi- mini- ing maxi- mini- im- (Hz) mum mum images mum mumimages mum mum images mum mum images mum mum images mum mum ages 1001.19 0.36 X 1.11 0.40 X 0.99 0.47 X 0.95 0.51 X 0.88 0.59 X 0.78 0.74 ◯500 0.77 0.74 ◯ 0.76 0.73 ◯ 0.76 0.74 ◯ 0.77 0.74 ◯ 0.78 0.74 ◯ 0.760.74 ◯ 1k 0.76 0.73 ◯ 0.76 0.74 ◯ 0.77 0.73 ◯ 0.77 0.73 ◯ 0.76 0.74 ◯0.78 0.75 ◯ 2k 0.76 0.74 ◯ 0.77 0.74 ◯ 0.76 0.74 ◯ 0.76 0.73 ◯ 0.77 0.73◯ 0.77 0.74 ◯ 5k 0.76 0.73 ◯ 0.76 0.74 ◯ 0.77 0.74 ◯ 0.76 0.73 ◯ 0.770.74 ◯ 0.76 0.74 ◯

Tables 6 and 7 show:

-   (1) Images can be properly formed when the strength of alternating    electric field to be applied to the developer in the medium, i.e.    the strength of alternating electric field to be applied to the    space in the developer-accommodating cell, is 0.5 V/μm or more;-   (2) Images can be properly formed when the frequency of alternating    electric field to be applied is 5 kHz or less; and-   (3) Images can be properly formed when the value of (frequency    [KHz]×time [ms] for application of alternating electric field)=20 or    more.

Good images were formed under these conditions using the medium D1y bythe image forming apparatus of FIG. 18(A) as described above.

The strength of alternating electric field to be applied forinitialization may be the same when using the image display mediums ofthe same type as the mediums 11, 12, 12′, 13, 15 and 15′.

In respect of the medium D2y of the same type as the medium 11, thesecond electrode 115 was set to carry a ground potential using the imageforming apparatus shown in FIGS. 1 and 2. The independent electrodes 114a corresponding to the pixels to be displayed in black were suppliedwith a negative voltage, and the independent electrodes 114 acorresponding to the pixels to be displayed in white were supplied witha positive voltage. In this way, the independent electrodes 114 a weresupplied with a voltage corresponding to the display data for imagedisplay. Good images were obtained by applying an alternating electricfield as in the case of the medium D1y before image display.

In the case of the medium D1y, for adjustment of alternating electricfield strength E to 0.5 V/μm, a voltage of about 60 V (Vpp about 120 V)was needed to be applied to the charging brush ch1 from the power sourcePwa. If change can be made of any one of the specific dielectricconstant of the substrate and the thickness thereof, and the specificdielectric constant of the cell layer and the thickness thereof, thevoltage value to be applied to the charging brush ch1 may becorrespondingly changed to achieve an electric field strength of 0.5V/μm or more.

(IV) Examples of Applying a Magnetic Stirring Force to the DeveloperBefore and/or in Image Display Step

<Developing Particles and Developer>

White Developing Particles WP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofthermoplastic polyester resin (softening point: 121° C., Tg: 67° C.), 40parts by weight of titanium oxide (Ishihara Sangyo Kaisha, Ltd.: CR-50)and 5 parts by weight of salicylic acid-zinc complex(minus-charge-controlling agent, Orient Chemical Co., Ltd.: BontronE-84) . The mixture was further mixed by a twin-screw extruder and thencooled. The mixture was roughly pulverized, then pulverized by a jetmill and classified with wind to obtain a white fine powder which has avolume average particle size of 12 μm. To the white fine powder wasadded 0.3 parts by weight of hydrophobic silica particles (Nihon AerosilCo., Ltd.: Aerosil R-972). The mixture was mixed by a Henschel mixer toprepare white developing particles WP.

Black Developing Particles BP

In a Henschel mixer were thoroughly mixed 100 parts by weight ofstyrene-n-butyl methacrylate resin (softening point: 132° C., Tg: 65°C.), 2 parts by weight of carbon black (Lion Oil & Fat Co., Ltd.:Ketchen Black), 1.5 parts by weight of silica (Nihon Aerosil Co., Ltd.:# 200) and 500 parts by weight of magnetic powder containing magnetite(RB-BL, Titan Kogyo Co., Ltd.). The mixture was further mixed by akneader and then cooled.

The mixture was roughly pulverized by a feather mill, then finelypulverized by a jet mill and classified with wind to obtain blackparticles BP which have a volume average particle size of 25 μm.

Preparation of Developer DL

The white particles WP (30 g) and the black particles BP (70 g) wereplaced into a polyethylene bottle. The bottle was rotated by a ball millpedestal to perform the kneading and mixing for 30 minutes to obtain adeveloper. The white developing particles WP were negatively charged andblack developing particles BP were positively charged in the developer.

<Reversible Image Display Medium D1z>

A reversible image display medium of the same type as the medium 14shown in FIG. 8(A) was prepared as follows.

A transparent PET (polyethylene terephthalate) film was molded bythermal pressing molding method to give a plurality of continuouspartition walls 113 a on a base portion of 25 μm thickness correspondingto the first substrate 111. The continuous partition wall 113 a had awall thickness (width) α (30 μm), a height h (100 μm) and a space pt(500 μm) between adjacent partition walls.

The developer DL was placed into each of the continuous groove-likecells 116 between the neighboring partition walls 113 a on the substrate111. The developer was placed into the continuous groove-like cell at avolume ratio of 30 vol. % based on the volume of the continuousgroove-like cell.

A photo-curing adhesive was applied to a small thickness only to the topof the partition walls 113 a of the substrate 111. Then, a PET film of25 μm thickness containing carbon black as the second substrate 112 wasclosely laid on the adhesive over its internal surface. The adhesive wascured by UV irradiation to adhere the film.

Thereafter, the peripheries of the first and second substrates 111, 112were heat-sealed.

In this way, a medium D1z of the type shown in FIG. 8(A) was produced.

<Image Display Using the Medium D1z>

Using the medium D1z, image display was performed by the image formingapparatus of FIG. 21 (A)

The surface of the photosensitive drum PC was uniformly charged by thecharger CH to −900V. The images exposure was performed to the chargedregion by the image exposing device EX and the surface potential in theexposed region was reduced to about −50 V to form an electrostaticlatent image E1.

On the other hand, the medium D1z fed was initialized by the magnetplate MG3. Then the surface of the medium D1z was uniformly charged bythe charger CRH before image display to have a polarity of about +400 Vopposite to the charged polarity of the surface of photosensitive drum.

Subsequently, the medium D1z was passed between the drum PC and theopposite electrode roller (transfer roller) R1 in synchronization withthe electrostatic latent images E1 formed on the drum PC. The roller R1of about 10⁸Ω in resistance was used and a transfer bias voltage ofabout +1300V was applied to the roller R1 from the power source PW1.

In this way, the electrostatic latent image was transferred to thesurface of the medium D1z at about −300V in the image portion (blackportion) and at about +300V in the background portion (white portion)

The medium D1z carrying the electrostatic latent image as describedabove was passed above the magnet plate MG2. Thereby a stirring forcewas affected on the developer DL containing magnetic developingparticles BP in the medium by the oscillating magnetic field, resultingin facilitated movement of developing particles for image display.

In this way, good images were obtained.

<Reversible Image Display Medium D2z>

A reversible image display medium of the same type as the medium 11having electrodes as shown in FIG. 1 was prepared as follows.

A photoresist was applied to a transparent ITO film on the entiresurface of a transparent PET (polyethylene terephthalate) film of 50 μmin thickness as the first substrate 111. Over the photoresist was laid aphotomask opened in a specified pattern, followed by exposure to light,development and etching. Then the remaining photoresist was peeled offfor removal. A first electrode pattern was formed in which square pixelelectrodes 114 a (see FIG. 4) measuring 1 mm and 1 mm were arranged asin a checkerboard with the squares spaced from each other by 0.1 mm insuch a way that lead portions 110 (see FIG. 4) are disposed between andconnected to the pixel electrodes.

A resist was repeatedly applied to increased thickness to other portionsthan the square electrodes 114 a on the first substrate 111 to form thegrid-like partition member 113 (see FIG. 3). The partition walls 113 aforming the partition member 113 (see FIG. 3) had a thickness (width) α(0.1 mm), a height h (100 μm), and a wall space pt (1 mm) (correspondingto one side of the independent electrode 114 a).

An ITO film was formed as the second electrode 115 by a sputteringmethod to a thickness of 500 Å over the entire surface of the secondsubstrate 112 formed of a transparent PET film of 50 μm in thickness.

Then, the developer DL was placed into each cavity surrounded with thepartition wall 113 a of the first substrate 111. The developer wasplaced into the cavity in a volume proportion of 30 vol. % based on thevolume of the cavity.

A photo-curing adhesive 119 a (see FIG. 1) was applied to a smallthickness only to the top of the walls 113 a after which the ITOelectrode 115 side of the second substrate 112 was closely laid on thetop. Then, the adhesive was cured by UV irradiation.

Thereafter, the peripheries of the first and second substrates 111, 112were sealed by an epoxy resin adhesive 119 b (see FIG. 1).

In this way, the medium D2z of the type shown in FIG. 1 was produced.

(Image Display Using the Medium D2z)

In image display using the medium D2z, the image forming apparatus shownin FIG. 19 was used and the second electrode 115 was set to carry aground potential. A negative voltage was applied to the independentelectrodes 114 a which correspond to the pixels to be displayed inblack, while a positive voltage was applied to the independentelectrodes 114 a which correspond to the pixels to be displayed inwhite. In this manner, each independent electrode 114 a was suppliedwith a voltage corresponding to the display data to display images. Thedeveloper was initialized by oscillating the magnet plate GM1 beforeimage display, while the developing particles were made easily movableby oscillating the magnet plate GM1 in image display. In this way, goodimages were obtained.

Now description was given to experiments for confirming the stirringeffect of the developer in the medium which was achieved by the magneticfield-generating member.

The experiments were carried out using the medium D1z and holding thesame between a pair of electrodes while the magnet plate is providedoutside the substrate 112 on a side opposite to the image observationside. The magnet plate was reciprocatingly oscillated in parallel withthe substrate 112, and N and S magnetic poles were alternately arrangedin the oscillating direction.

Each experiment was made in the following manner (Step 1 and Step 2). Ineach experiment, use was made of a magnet plate different in magneticflux density acting on the medium D1z in Step 2 while the number ofmagnetic poles passing through parts of the developer DL was changed.

Step 1

The electrode on the side of the substrate 112 opposed to the imageobservation side was set to a ground potential. In this state, −200 Vwas applied to the electrode on the side of the substrate 111 on theimage observation side, while the magnet plate was reciprocatinglyoscillated in a sufficient degree in parallel with the substrate 112.Thereby images were formed with black magnetic magnetic particles BPlocally distributed on the image observation side. The images were blackin their entirety when seen from the image observation side.

Step 2

After Step 1, the electrode on the side of the substrate 112 was set toa ground potential. In this state, +200 V was applied to the electrodeon the side of the substrate 111, while the magnet plate of the sametype as above which was provided similarly was reciprocatinglyoscillated. Thereby images were formed with white developing particlesWP locally distributed on the image observation side.

The stirring effect on the developer was evaluated in each experiment.The results were evaluated in terms of the reflection density in thewhite portion when seen from the image observation side after Step 2. Ifthe maximum reflection density in white portion is 0.4 or less and adifference between the maximum reflection density and the minimumreflection density in white portion is 0.2 or less, it was visuallyexcellent. In this case, the developer stirring effect was deemed to besufficient (o), and if not so, the developer stirring effect was deemedto be insufficient (x).

The image density was measured by a reflection densitometer (product ofKonica Corporation, Sakura DENSITMETER PDA-65).

The evaluation results are shown in Table 8 together with magnetic fluxdensity against the medium D1z of the magnet plate used in Step 2 andthe number of magnetic poles of the plate passing through parts of thedeveloper.

It is evident from Table 8 that desirably the magnetic flux density ofthe magnet plate against the medium was 50 gausses or more and at leasttwo magnetic poles pass through parts of the developer for smoothmovement of developing particles. TABLE 8 number of passing magneticpoles 1 number of passing magnetic poles 2 number of passing magneticpoles 3 image image image mag- image irregularity image irregularityimage irregularity netic density maximum − density maximum − densitymaximum − flux maxi- mini- minimum maxi- mini- minimum stir- maxi- mini-minimum density mum mum (white portion) stirring mum mum (white portion)ring mum mum (white portion) stirring (gauss) {circle over (1)} {circleover (2)} {circle over (1)} − {circle over (2)} effect {circle over (1)}{circle over (2)} {circle over (1)} − {circle over (2)} effect {circleover (1)} {circle over (2)} {circle over (1)} − {circle over (2)} effect30 1.67 1.50 0.17 X 0.84 0.49 0.35 X 0.62 0.37 0.25 X 40 1.63 1.52 0.11X 0.67 0.40 0.27 X 0.48 0.24 0.24 X 50 1.50 1.10 0.40 X 0.37 0.30 0.07 ◯0.23 0.21 0.02 ◯ 60 1.42 1.01 0.41 X 0.35 0.31 0.04 ◯ 0.23 0.21 0.02 ◯100 1.42 0.97 0.45 X 0.36 0.32 0.04 ◯ 0.22 0.20 0.02 ◯ 200 1.47 0.920.55 X 0.33 0.31 0.02 ◯ 0.23 0.21 0.02 ◯ 300 1.33 0.83 0.50 X 0.32 0.300.02 ◯ 0.22 0.20 0.02 ◯

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A method for displaying an image, comprising the steps of: providinga reversible image display medium comprising; two substrates opposed toeach other with a gap therebetween; one or more developer accommodatingcells formed between the two substrates, each having a peripherysurrounded by a partition wall; and a dry developer contained in each ofthe cells, the dry developer containing at least two kinds offrictionally chargeable dry developing particles having differentchargeable polarities and different optical reflection densities; anddisplaying an image by driving the frictionally charged developingparticles having different chargeable polarities in an electrostaticfield corresponding to the image to be displayed, wherein in the imagedisplay step, strength of the electric field to be applied to thedeveloper is 0.3 V/μm to 3.0 V/μm.
 2. The method according to claim 1,wherein at least one kind of the developing particles among the drydeveloping particles are magnetic particles, and a magnetic stirringforce is applied to the developer by a magnetic field in relation todriving the developing particles in the electrostatic field. 3-7.(canceled)
 8. A method for displaying an image, comprising the steps of:providing a reversible image display medium comprising; two substratesopposed to each other with a gap therebetween; one or more developeraccommodating cells formed between the two substrates, each having aperiphery surrounded by a partition wall; and a dry developer containedin each of the cells, the dry developer containing at least two kinds offrictionally chargeable dry developing particles having differentchargeable polarities and different optical reflection densities;displaying an image by applying from outside an electrostatic fieldcorresponding to the image to be displayed to the frictionally chargeddeveloping particles having different chargeable polarities to drive thedeveloping particles for image display; and charging a surface of theimage display medium on image observation side to carry a potentialholding the displayed image after completion of application of theelectrostatic field.
 9. (canceled)
 10. The method according to claim 8,wherein the potential holding the displayed image is 100 V or less interms of absolute value.
 11. A method for displaying an image,comprising the steps of: providing a reversible image display mediumcomprising; two substrates opposed to each other with a gaptherebetween; one or more developer accommodating cells formed betweenthe two substrates, each having a periphery surrounded by a partitionwall; and a dry developer contained in each of the cell(s), the drydeveloper containing at least two kinds of frictionally chargeable drydeveloping particles having different chargeable polarities anddifferent optical reflection densities; initializing the reversibleimage display medium by stirring the developer in the image displaymedium before image display on the display medium; and displaying animage by driving the frictionally charged dry developing particleshaving different chargeable polarities within the above-initializedreversible image display medium in an electrostatic field correspondingto the image to be displayed.
 12. The method according to claim 11,wherein the initialization is conducted by application of an alternatingelectric field to the developer in the medium.
 13. The method accordingto claim 12, wherein strength of the alternating electric field to beapplied to the developer is 0.5 V/μm or more.
 14. The method accordingto claim 12, wherein frequency of the alternating electric field to beapplied to the developer is 5 kHz or less.
 15. The method according toclaim 12, wherein the application of alternating electric field to thedeveloper in the medium is performed to satisfy a condition:(frequency[Hz] of alternating electric field.times.time[second(s)] forapplication of alternating electric field)=20 or more.
 16. An imageforming apparatus which displays an image using a reversible imagedisplay medium comprising: two substrates opposed to each other with agap therebetween; one or more developer accommodating cells formedbetween the two substrates, each having a periphery surrounded by apartition wall; and a dry developer contained in each of the cell(s),the dry developer containing at least two kinds of frictionallychargeable dry developing particles having different chargeablepolarities and different optical reflection densities, the apparatuscomprising: a device for initializing the reversible image displaymedium by stirring the developer in the image display medium beforeimage display on the medium; and an image forming portion for displayingan image by driving the frictionally charged developing particles havingdifferent chargeable polarities within the initialized medium in anelectrostatic field corresponding to the image to be displayed.
 17. Theimage forming apparatus according to claim 16, wherein the initializingdevice is one in which the developer is stirred by application of analternating electric field to the developer in the reversible imagedisplay medium.
 18. The image forming apparatus according to claim 17,wherein the initializing device applies the alternating electric fieldhaving an electric field strength of 0.5 V/μm or more to the developer.19. The image forming apparatus according to claim 17, wherein theinitializing device applies the alternating electric field having afrequency of 5 kHz or less to the developer.
 20. The image formingapparatus according to claim 17, wherein the initializing device appliesthe alternating electric field to the developer to satisfy a condition:(frequency[Hz] of alternating electric field) X (time[second(s)] forapplication of alternating electric field)=20 or more. 21-36. (canceled)